12.4 remote POP3 mailboxes
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## 12.4 remote POP3 mailboxes

Kerio MailServer can retrieve messages from POP3 boxes at different mailservers and deliver them to local mailboxes or send them to different email addresses.

Retrieving POP3 mailboxes is controlled only by a scheduler (see chapter 20 Scheduling). It is important to realize that mail will not be downloaded from remote POP3 accounts automatically when a client connects to his/her Kerio MailServer mailbox or sends an email.

Downloading of POP3 accounts disables antispam features which depend on reception of email by SMTP (typically DNS blacklists and check of Caller ID and SPF sender servers. Configuration and features of antispam filters are focused in chapter 13 Antispam control of the SMTP server.

### Defining Remote Mailboxes

Remote mailboxes from which email should be retrieved can be defined in the Configuration → POP3 Download section using the Accounts tab.

Figure 12.8. Remote POP3 download

Use the Add button to display a dialog box that allows users to add a new account (a remote mailbox). With the General tab, set the basic parameters for accessing the mailbox and the delivery method for the downloaded email.

Figure 12.9. Defining Remote Mailboxes

POP3 server

  The DNS name or IP address of the POP3 server where the mailbox is located.

Username and Password

  The username and password for the mailbox

Description

  Any text description of the POP3 account

Deliver to address

  All messages from the mailbox will be sent to one address. Here you can enter a local user, a local group, an alias or an external email address. You can choose the local user or group from a list using the Select button.

  This dialog allows to search for a specified string and specify the settings for the case-sensitivity. These options make the search faster, especially when searching through too many users and groups in the domain.

Use sorting rules

  Messages from this mailbox will be sorted according to the sorting rules (see below).

Preferred header

  The primary header entry that will be used for sorting. Here you can specify a header entry (the name of the header without a colon) or choose one from the list (X-Envelope-To, Received or Delivered-To). If the entry is not found in the mail header or no address complies with any rule, other header entries are searched — `Resent-To` and `Resent-Cc`, `To` and `Cc`. If an address is not found in these entries the message will be delivered according to an implicit rule (described below) or will be discarded.

Drop duplicate messages

  If this option is enabled, and identical copies of one message are stored on a remote mailbox, only one copy will be downloaded (the others will be dropped).

  Messages are duplicated when a message with more recipients (included in the domain) in the header is delivered into the domain mailbox. In such a case, the message is delivered to the mailbox that many times how many recipients were originally specified. These messages differ only in their SMTP envelope. However, the envelope is cut out when the message is saved in the mailbox. All copies of a message stored in the mailbox will be identical. During standard POP3 sorting each recipient receives all the messages as his/her address is included (this means that each recipient receives the same number of copies as number of recipients). Dropping duplicate messages ensures that each recipient receives one of the copies only.

You can define the following parameters with the Advanced tab:

Figure 12.10. More detailed settings for downloading POP3 mailboxes

Use SSL

  The connection with the POP3 server will be secured (encrypted) by SSL.

SSL Mode

  The security method for communication with the POP3 server. Options: Special port (the SSL connection will be established on a port different from a standard POP3 port) or STLS command (first, a non-encrypted connection will be made and once it is established it will be switched to an encrypted mode using the STLS command). Contact the POP3 server administrator for more information about securing communication with the POP3 server.

POP3 authentication

  The POP3 server authentication method: Plain (the password is sent in its normal form) or APOP (the password is encrypted to prevent tapping and misuse). Contact the POP3 server administrator for more information.

Per Session Download Limits

  - Total message size — this entry enables specification of a maximal total size of messages downloaded within one POP3 session. The zero value means that no limit has been set.

  - Max message count — The maximum number of messages that will be downloaded during one connection (if there are more messages at the server they will be downloaded in the next session). The zero value means that no limit has been set.

  The total message size limit protects the user from repeated downloads of identical messages in cases where POP3 session was interrupted.

  The main reason is the principle of POP3 protocol. On the server, messages to be deleted are not physically removed until a successful disconnection by the QUIT command. If the POP3 session is interrupted, messages are not removed and the server downloads them again within the following POP3 session. Setting of these limits therefore helps to control of data flowing in repeated sessions.

For temporary remove of appropriate rules use matching fields next to the rule definitions.

### Sorting Rules

Sorting rules define how messages downloaded from a remote POP3 mailbox will be delivered to and divided between local users or forwarded to external email addresses. Use the Sorting Rules tab to define sorting rules.

Figure 12.11. Sorting Rules

Use the Add button to add a new sorting rule:

Sort Address

  Email address that will be searched for in the selected message header entry. It must be complete; a substring is not acceptable.

Deliver To

  This entry defines the recipient of the message complying with the rule. Here you can specify:

  Figure 12.12. Sorting Rule dialog

  - local user or group of users — local users/groups of users can be selected using the Select button,

  - alias — enter an appropriate alias,

  - external email address — any other email address.

  ### Note

  To deliver messages to groups, you must assign addresses to these groups (or you can create an alias). For details refer to chapter 9 User groups.

Description

  A commentary on a sorting rule (e.g. purpose explanation)

For temporary remove of appropriate rules use matching fields next to the rule definitions.

### Special Sorting Rules

In sorting rules you can also define rules in this format:

- `*` → `address` (implicit rule) Email messages not complying with any rule will be delivered to this user (group). If this rule is not defined, such messages will be discarded.

- `*@domain.com` → `*@anotherdomain.com` All messages containing the specified domain will be forwarded to another specified domain.

No other usage of the asterisk character (e.g. for completing a part of an address) is allowed.

Example of wildcard usage

  In this section you will find examples of how wildcard can be used for the simplest settings of sorting rules. The configuration consists of the following rules:

  - The first rule sorts messages by alias settings and by addresses of user accounts.

    `*@company.com → *@company.com`

  - The second rule sorts messages which, by any reason, cannot be sorted to any particular user account.

    `* → admin@company.com`

  ### Note

  If any other rule is placed above these rules, it will be processed before them. Rules are always processed in the following order:

  1. `address@domain`

  2. `*@domain`

  3. `*`

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