Electronic Mail Service (SMTP/POP3-IMAP4)

The electronic mail service is a store and forward method [1] to compose, send, store and receive messages over electronic communication systems.

[1]	Store and forward: Telecommunication technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station.

How electronic mail works through the Internet

Diagram where Alice sends an email to Bob

The diagram depicts a typical event sequence that takes place when Alice writes a message to Bob using her Mail User Agent (MUA).

1. Her MUA formats the message in email format and uses the Simple Mail Transfer Protocol (SMTP) to send the message to the local Mail Transfer Agent (MTA).
2. The MTA looks at the destination address provided in the SMTP (not from the message header), in this case bob@b.org, and resolves a domain name to determine the fully qualified domain name of the destination mail exchanger server (MX record that was explained in the DNS section).
3. smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the user bob.
4. Bob receives the message through his MUA, which picks up the message using Pop Office Protocol (POP3).

There are many alternative possibilities and complications to the previous email system sequence. For instance, Bob may pick up his email in many ways, for example using the Internet Message Access Protocol (IMAP), by logging into mx.b.org and reading it directly, or by using a Webmail service.

The sending and reception of emails between mail servers is done through SMTP but the users pick up their email using POP3, IMAP or their secure versions (POP3S and IMAPS). Using these protocols provides interoperability among different servers and email clients. There are also proprietary protocols such as the ones used by Microsoft Exchange and IBM Lotus Notes.

POP3 vs. IMAP

The POP3 design to retrieve email messages is useful for slow connections, allowing users to pick up all their email all at once to see and manage it without being connected. These messages are usually removed from the user mailbox in the server, although most MUAs allow to keep them on the server.

The more modern IMAP, allows you to work on-line or offline as well as to explicitly manage server stored messages. Additionally, it supports simultaneous access by multiple clients to the same mailbox or partial retrievals from MIME messages among other advantages. However, it is a quite complicated protocol with more server work load than POP3, which puts most of the load on the client side. The main advantages over POP3 are:

• Connected and disconnected modes of operation.
• Multiple clients simultaneously connected to the same mailbox.
• Access to MIME message parts and partial fetching.
• Message state information using flags (read, removed, replied, ...).
• Multiple mailboxes on the server (usually presented to the user as folders) allowing to make some of them public.
• Server-side searches
• Built-in extension mechanism

Both POP3 and IMAP have secure versions, called respectively POP3S and IMAPS. The difference with its plain version is that they use TLS encryption so the content of the messages cannot be eavesdropped.

SMTP/POP3-IMAP4 server configuration with eBox

Setting up an email system service requires to configure an MTA to send and receive emails as well as IMAP and/or POP3 servers to allow users to retrieve their mails.

To send and receive emails Postfix [2] acts as SMTP server. The email retrieval service (POP3, IMAP4) is provided by Dovecot [3]. Both servers support secure communication using SSL.

Footnotes

[2]	Postfix The Postfix Home Page http://www.postfix.org .

[3]	Dovecot Secure IMAP and POP3 Server http://www.dovecot.org .

Receiving and relaying mail

In order to understand the mail system configuration, a distinction must be made between receiving mail and relaying mail.

Reception is when the server accepts a mail message whose recipients contains an account that belongs to any of his virtual mail domains. Mail can be received from any client which is able to connect to the server.

On the other hand, relay is done when the mail server receives a message whose recipients do not belong to any of his managed virtual mail domains, thus requiring forwarding the message to other server. Mail relay is restricted, otherwise spammers could use the server to send spam over the Internet.

eBox allows mail relay in two cases:

1. an authenticated user
2. a source address that belongs to a network object which has a allowed relay policy

General configuration

Through Mail ‣ General ‣ Mail server options ‣ Authentication, you can manage the authentication options. The following options are available:

TLS for SMTP server:

Force the clients to connect to the mail server using TLS encryption, thus avoiding eavesdropping.

Require authentication:

This setting enables the authentication usage. A user must use his email address and his password to identify himself, authenticated users can relay mail through the server. An account alias cannot be used to authenticate.

In the Mail ‣ General ‣ Mail server options ‣ Options section you may configure the general settings for the mail service:

Smarthost to send mail:

Domain name or IP address of the smarthost. You could also specify a port appending the text :[port_number ] after the address. The default port is the standard SMTP port, 25.

If this option is set eBox will not send its messages directly, but each received email will be forwarded to the smarthost without keeping a copy. In this case, eBox would be an intermediary between the user who sends the email and the server which is the real message sender.

Smarthost authentication:

Whether the smarthost requires authentication using a user and password pair or not.

Server mailname:

This sets the visible mail name of the system, it will be used by the mail server as the local address of the system.

Postmaster address:

The postmaster address by default is aliased to the root system user but it could be set to any account, belonging to any of managed virtual mail domains or not.

This account is intended to be a standard way to reach the administrator of the mail server. Automatically-generated notification mails will typically use postmaster as reply address.

Maximum mailbox size allowed:

Using this option you could indicate a maximum size in MB for any user mailboxes. All mail which surpasses the limit will be rejected and the sender will be emailed a notification. This setting could be overridden for any user in the Users and Groups ‣ Users page.

Maximum message size accepted:

Indicates, if necessary, the maximum message size accepted by the smarthost in MB. This is enforced regardless of any mailbox size limit.

Expiration period for deleted mails:

If you enable this option those mail messages which are in the users’ trash folder will be deleted when their dates passes the day limit.

Expiration period for spam mails:

This option applies the same way as above option but regarding to the users’ spam folder.

In order to configure the mail retrieval services go to the Mail retrieval services section. There eBox may be configured as POP3 and/or IMAP server, their secure versions POP3S and IMAPS are available too. Also the retrieve email for external accounts and ManageSieve services could be enabled in this section, we will discuss those services in Mail retrieval from external accounts section.

In addition to this, eBox may be configured to relay mail without authentication from some network addresses. To do so, you can add relay policies for network objects through Mail ‣ General ‣ Relay policy for network objects. The policies are based on the source mail client IP address. If the relay is allowed from an object, then each object member may relay emails through eBox.

Warning

Be careful when using an Open Relay policy, i.e., forwarding email from everywhere, since your mail server will probably become a spam source.

Finally, the mail server may be configured to use a content filter for their messages [4]. To do so, the filter server must receive the message from a fixed port and send the result back to another established port where the mail server is bound to listen the response. Through Mail ‣ General ‣ Mail filter options, you may choose a custom server or eBox as mail filter.

[4]	In Mail Filter section this topic is deeply explained.

Email account creation through virtual domains

In order to set up an email account with a mailbox, a virtual domain and a user are required. From Mail ‣ Virtual Mail Domains, you may create as many virtual domains as you want. They provide the domain name for email accounts for eBox users. Moreover, it is possible to set aliases for a virtual domain. It does not make any difference to send an email to one virtual domain or any of its aliases.

In order to set up email accounts, you have to follow the same rules applied configuring any other user-based service . From Users and Groups ‣ Users ‣ Edit Users ‣ Create mail account. You select the main virtual domain for the user there. If you want to establish to the user more than a single email address, you can create aliases. Behind the scenes, the email messages are kept just once in a mailbox. However, it is not possible to use the alias to authenticate, you always have to use the real account.

Note that you can decide whether an email account should be created by default when a new user is added or not. You can change this behaviour in Users and Groups ‣ Default User Template ‣ Mail Account.

Likewise, you may set up aliases for user groups. Messages received by these aliases are sent to every user of the group which has an email account. Group aliases are created through Users and Groups ‣ Groups ‣ Create alias mail account to group. The group aliases are only available when, at least, one user of the group has an email account.

You may define alias to external accounts as well. The mail sent to that alias will be forwarded to the external account. This kind of aliases are set on a virtual domain basis and does not require any email account and could be set in Mail ‣ Virtual Domains ‣ External accounts aliases.

Queue Management

From Mail ‣ Queue Management, you may see those email messages that haven’t already been delivered. All the information about the messages is displayed. The allowed actions to perform are: deletion, content viewing or retrying sending (re-queuing the message again). There are also two buttons to delete or requeue all messages in queue.

Mail retrieval from external accounts

You could configure eBox to retrieve email messages from external accounts, which are stored in external servers, and deliver them to the user’s mailboxes. In order to configure this you have to enable this service in Mail ‣ General ‣ Mail server options ‣ Retrieval services section. Once it is enabled, the users will have their mail fetched from their external accounts and delivered to their internal account’s mailbox. Each user can configure its external accounts through the user corner [5]. The user must have a email account to be able to do this. The external servers are pooled periodically so email retrieval is not instantaneous.

To configure its external accounts, a user have to login in the user corner and click on Mail retrieval from external mail accounts in the left menu. In this page a list of user’s external accounts is shown, the user can add, edit and delete accounts. Each account has the following fields:

External account:

The username or the mail address required to login in the external mail retrieval service.

Password:

Password to authenticate the external account.

Mail server:

Address of the mail server which hosts the external account.

Protocol:

Mail retrieval protocol used by the external account, it may be one of the following: POP3, POP3S, IMAP or IMAPS.

Port:

Port used to connect to the external mail server.

For retrieving external emails, eBox uses the Fetchmail [6] software.

Footnotes

[5]	The user corner configuration is explained in User Corner section

[6]	Fetchmail The Fetchmail Home Page http://fetchmail.berlios.de/ .

Sieve scripts and ManageSieve protocol

The Sieve language [7] allows the user to control how his mail messages are delivered, so it is possible to classify it in IMAP folders, forward it or use a vacation message among other things.

The ManageSieve is a network protocol that allows the users to easily manage their Sieve scripts. To be able to use ManageSieve, it is required an email client that understands this protocol. [8]

To enable ManageSieve in eBox you have to turn on the service in Mail ‣ General ‣ Mail server options -> Retrieval services and it could be used by all the users with email account. In addition to this, if ManageSieve is enabled and the webmail [9] module in use, a management interface for Sieve scripts will be available in the webmail interface.

The ManageSieve authentication is done with the email account of the user and its password.

Sieve scripts of an account are executed regardless of the ManageSieve protocol option value.

Footnotes

[7]	For more info check out this page http://sieve.info/ .

[8]	See a list of clients in this page http://sieve.info/clients

[9]	The webmail module is explained in WebMail service chapter.

Email client configuration

Unless users may use email only through the webmail or the egroupware webmail application, users would like to configure their email clients to use eBox’s mail server. The values of the required parameters would depend on the exact configuration of the module.

Please note that different email clients could use other names for these parameters, so due to the great number of clients available this section is merely guidance.

SMTP parameters

SMTP server:

Enter the address of your eBox server. It could be either an IP address or a domain name.

SMTP port:

25, if you are using TLS you could instead use the port 465.

Secure connection:

Select TLS if you have enabled TLS for SMTP server:, otherwise select none. If you are using TLS please read the warning below about SSL/TLS.

SMTP username:

Use this if you have enabled Require authentication. Use as username the full email address of the user, don’t use the username nor any of his mail aliases.

SMTP password:

It is the user password.

POP3 parameters

You can only use POP3 settings when POP3 or POP3S services are enabled in eBox.

POP3 server:

Enter your eBox address likewise in the SMTP parameters section above.

POP3 port:

110 or 995 if you are using POP3S.

Secure connection:

Select SSL if you are using POP3S, otherwise none. If you are using POP3S please read the warning below about SSL/TLS.

POP3 username:

Full email address, as above avoid the user name or any of his email aliases.

POP3 password:

User’s password.

IMAP parameters

IMAP configuration could be only used if either IMAP or IMAPS services are enabled. As you will see the parameters are almost identical to POP3 parameters.

IMAP server:

Enter your eBox address likewise in the SMTP parameters section above.

IMAP port:

443 or 993 if you are using IMAPS.

Secure connection:

Select SSL if you are using IMAPS, otherwise none. If you are using IMAPS please read the warning below about SSL/TLS.

IMAP username:

Full email address, as above avoid the user name or any of his email aliases.

IMAP password:

User’s password.

Warning

In client implementations there are some confusion about the use of SSL and TLS protocols. Some clients use SSL to mean that they will try to connect with TLS, others use TLS as a way to say that they will try to connect to the secure service through a port used normally by the plain version of the protocol.. In fact in some clients you will need to try both SSL and TLS modes to find which one works. You have more information about this issue in this page http://wiki.dovecot.org/SSL , from the dovecot’s wiki.

ManageSieve client parameters

To connect to ManageSieve, you will need the following parameters:

Sieve server:

The same that your IMAP or POP server.

Port:

4190, be warned that some applications use, mistakenly, the port number 2000 as default for ManageSieve.

Secure connection:

Set to true

Username:

Full mail address, as above avoid the user name or any of his email aliases.

Password:

User’s password. Some clients allows you to select the same authentication than your IMAP or POP3 account if this is allowed, select it.

Catch-all account

A catch-all account is those which receives a copy of all the mail sent and received by a mail domain. eBox allows you to define a catch-all account for every virtual domain; to define it you must go to Mail ‣ Virtual Mail Domains and then click in the Settings cell.

All the messages sent and received by the domain will be emailed as Blind Carbon Copy (BCC) to the defined address. If the mail to the catch-all address bounces, it will be returned to the sender.

Practice example

Set up a virtual domain for the mail service. Create a user account and a mail account within the domain for that user. Configure the relay policy to send email messages. Send a test email message with the new account to an external mail account.

1. Action: Log into eBox, access Module status and enable Mail by checking its checkbox in the Status column. Enable Network and Users and Groups first if they are not already enabled.

   Effect:

   eBox requests permission to overwrite certain files.

2. Action: Read the changes of each of the files to be modified and grant eBox permission to overwrite them.

   Effect:

   The Save changes button has been enabled.

3. Action: Go to Mail ‣ Virtual Mail Domains and click Add new to create a new domain. Enter the name in the appropriate field.

   Effect:

   eBox notifies you that you must save changes to use this virtual domain.

4. Action: Save the changes.

   Effect:

   eBox displays the progress while the changes are being applied. Once this is completed, you will be notified.

   Now you may use the newly created virtual mail domain.

5. Action: Enter Users and Groups ‣ Users ‣ Add User, fill up the user data and click the Create and Edit button.

   Effect:

   The user is added immediately without saving changes. The edition screen is displayed for the newly created user.

6. Action: (This action is only required if you have disable the automatic creation of email accounts in Users and Groups –> Default User Template –> Mail Account). Enter a name for the user mail account in Create mail account and create it.

   Effect:

   The account has been added immediately and options to delete it or add aliases for it are shown.

7. Action: Enter the Object ‣ Add new menu. Fill in a name for the object and press Add. Click on Members in the created object. Fill in again a name for the member and write the host IP address where the mail will be sent from.

   Effect:

   The object has been added temporarily and you may use it in other eBox sections, but it is not persistent until you save changes.

8. Action: Enter Mail ‣ General ‣ Relay policy for network objects. Select the previously created object making sure Allow relay is checked and add it.

   Effect:

   The Save changes button has been enabled.

9. Action: Save the changes

   Effect:

   A relay policy for that object has been added, which makes possible from that object to send e-mails to the outside.

10. Action: Configure a selected MUA in order to use eBox as SMTP server and send a test email message from this new account to an external one.

    Effect:

    After a brief period you should receive the message in your external account mailbox.

11. Action: Verify using the mail server log file /var/log/mail.log that the email message was delivered correctly.