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POP Module |
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- The CommuniGate Pro POP module implements a POP3 server. POP3 servers allow client applications
(mailers) to retrieve messages from account mailboxes using the
POP3 Internet protocol (STD0053, RFC1939, RFC1734, RFC1725) via TCP/IP networks.
The CommuniGate Pro POP module implements several protocol extensions, including the XTND XMIT
extension. Some mailers can employ this feature to submit messages to the CommuniGate Pro server.
The POP module activity statistics is available via the CommuniGate Pro SNMP agent.
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Post Office Protocol (POP3)
The Post Office Protocol allows computers to retrieve messages from mailboxes
on mail servers. A computer running a mailer (mail client) application connects
to the mail server computer and provides account (user) name and the password.
If access to the specified user account is granted, the mail application
sends protocol commands to the mail server. These protocol commands tell
the server to list all messages in the mailbox, to retrieve certain messages,
or to delete them. When a server receives a request to retrieve a message,
it sends the entire message to the mail client. The mail client may choose
to retrieve only the first part of the message.
The POP3 protocol does not support multi-mailbox accounts. If a client
application specifies a multi-mailbox (folder) account, the INBOX mailbox
is opened.
When the client application sends a request to delete a message from
the mailbox, the message is not deleted immediately, but it is marked by
the server. Only when the client application ends the session properly and
closes the connection, the marked messages are then removed.
The POP module supports the XTND XMIT extension of the POP protocol.
This extension allows users to submit messages via the POP protocol instead
of the SMTP protocol.
Configuring the POP module
-
Use a Web browser to configure the POP module. Open the Access page in
the WebAdmin Settings section (realm):
Use the Log Level setting to specify what kind of information the POP
module should put in the Server Log. Usually you should use the Major
(message transfer reports) or Problems (message transfer and non-fatal
errors) levels. But when you experience problems with the POP module, you
may want to set the Log Level setting to Low-Level or All
Info: in this case protocol-level or link-level details will be recorded
in the System Log as well. When the problem is solved, set the Log Level
setting to its regular value, otherwise your System Log files will grow
in size very quickly.
The POP module records in the System Log are marked with the POP
tag.
When you specify a non-zero value for the Maximum Number of Channels
setting, the POP module creates a so-called "listener". The module
starts to accept all POP connections that mail clients establish in order
to retrieve mail from your server. The setting is used to limit the number
of simultaneous connections the POP module can accept. If there are too
many incoming connections open, the module will reject new connections,
and the mail client should retry later.
By default, the POP module Listener accepts clear text connections on the
TCP port 110. The standard TCP port number for secure POP connections is 995, but it
is not enabled by default. Follow the
listener link to tune the POP Listener.
The POP module supports the STARTTLS command that allows client mailers to
establish a connection in the clear text mode and then turn it into a secure connection.
User Authentication
-
The POP module allows users to employ all authentication
methods supported with the CommuniGate Pro Server, as well as the APOP method.
Secure (encrypted) Access
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The POP module can be used to accept SSl/TLS (encrypted) connections from
user mailers (see the Listener configuration note above). Additionally, the
POP module supports the STLS command that allows client mailers to
establish plain text, unencrypted connections (using the regular TCP port 110),
and then start encrypted communications on those connections.
Special Features
-
Unlike many other POP servers, the CommuniGate Pro POP module does not
"lock" the mailbox it opens on a mail clients behalf. The open
mailbox can be used by other client applications at the same time. See the
Sharing section for the details.
Since the POP3 protocol was not designed to support these features,
the CommuniGate Pro POP module:
- shows only the messages that existed in the mailbox when the mailbox
was opened with the client mailer; all new messages received during this session
will be seen by the POP client only when it connects to the mailbox again;
- keeps zombies for the messages deleted during the current session;
the module shows them as messages of
a zero size, and the module reports an error when a client application
tries to retrieve a deleted message;
When a client mailer retrieves a message with the RETR command, the
message is marked with the "Seen" flag (this change is noticed
when using an IMAP client with the same mailbox). The TOP command that
allows a client POP mailer to retrieve only the first part of the message
does not set the Seen flag.
In order to support Microsoft® E-mail clients, the POP module
supports the non-standard "empty AUTH" command (the AUTH command
without parameters), returning the list of supported
SASL methods.
The XTND XMIT Extension
-
The CommuniGate Pro POP module implements the XTND XMIT protocol extension.
Mailer applications that support this extension (like Eudora®) can
submit messages to the Server via a POP connection.
This feature can be useful for mobile users that would be otherwise
unable to send their messages via CommuniGate Pro SMTP due to the Server
anti-spam protection. Submitting messages via POP can be more convenient
than using the "address-remembering"
scheme, since this method does not have time restrictions.
Notification Alerts
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The POP3 protocol does not provide any method to send a notifiction alert to the
client mailer. If an account has any pending alert message,
the CommuniGate Pro POP module simply rejects the connection request after the user is authenticated.
The returned error code contains the alert message text:
ALERT: alert message text
When the user repeats a connection attempt to the same account, the next pending alert
message is returned as an error - till all alert messages are sent to that user.
Accessing Additional Mailboxes
-
Unlike the IMAP protocol, the POP3 protocol was designed
to access only one account mailbox - the INBOX mailbox.
The POP module allows users to access any account mailbox by specifying the
mailbox name as a part of the account name. To access the mailbox mailboxname
in the accountname account, a user should specify the account name as:
mailboxname#accountname:
Account name
(specified in the mailer settings) |
Accessed Mailbox |
| jsmith | mailbox INBOX in the jsmith account |
| private#jsmith | mailbox private in the jsmith account |
| lists/info#jsmith@client1.com | mailbox lists/info in the jsmith account in the client1.com domain |
The POP module allows a user to access any mailbox in any other account (a foreign or
shared mailbox), as well as public mailboxes. See the Sharing
section for the details.
If a user can log into the accountname account and wants to access the mailbox
mailboxname in the otheraccount account, that user should specify the account name as:
~otheraccount/mailboxname#accountname:
Account name
(specified in the mailer settings) |
Accessed Mailbox |
| jsmith | mailbox INBOX in the jsmith account |
| ~public/announces#jsmith | the public mailbox announces |
| ~boss/INBOX#jsmith | mailbox INBOX in the boss account |
In all samples above, the user is authenticated as jsmith, using the jsmith account
password.
If the authenticated user does not have a right to delete messages in the selected mailbox,
the DELE protocol operations fail and an error code is returned to the user mailer.
The POP module can also use the Direct Mailbox Addressing
feature to open additional mailboxes.
Accessing Individual Mail in a Unified Account
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The POP module implements Unified Domain-Wide Account
filtering. As all access modules, the POP module uses the
Router to process the specified username.
If a client mailer specifies the abcdef@client1.com username (as used in
the example), the Router routes this address to the
Local account Cl1, and it returns abcdef as the local part of
the resulting address.
The POP module checks the local part returned by the Router, and if this string is not
empty, it performs filtering on the open mailbox: the module hides all mailbox messages
that do not have the X-Real-To header field (or other field specified
in the Local Delivery module settings), or do not have the specified string (individual
name) listed in that header field.
So, if the user has specified the abcdef@client1.com username, only the messages
originally routed to that particular address will be shown in the CL1 account mailbox.
If a user connects as Cl1, the same account mailbox will be opened, but since
the local part string will be empty in this case, all mailbox messages will be shown.
- Example:
- The Router line:
- client1.com = Cl1.local
- The first message is sent to:
- abcdef@client1.com
- It is stored in the Cl1 account INBOX with unique ID 101, and a header field is added:
- X-Real-To: abcdef
- The next message is sent to:
- xyz@client1.com
- It is stored in the Cl1 account INBOX with unique ID 102, and a header field is added:
- X-Real-To: xyz
- After these 2 messages are stored, POP sessions will show different views depending on the
user name specified:
-
S: +OK CommuniGate Pro POP Server is ready
C: USER Cl1
S: +OK, send pass
C: PASS mypassword
S: +OK 2 message(s)
C: UIDL
S: +OK
S: 1 101
S: 2 102
S: .
C: QUIT
S: +OK bye-bye
S: +OK CommuniGate Pro POP Server is ready
C: USER abcdef@client1.com
S: +OK, send pass
C: PASS mypassword
S: +OK 1 message(s)
C: UIDL
S: +OK
S: 1 101
S: .
C: QUIT
S: +OK bye-bye
S: +OK CommuniGate Pro POP Server is ready
C: USER xyz@client1.com
S: +OK, send pass
C: PASS mypassword
S: +OK 1 message(s)
C: UIDL
S: +OK
S: 1 102
S: .
C: QUIT
S: +OK bye-bye
S: +OK CommuniGate Pro POP Server is ready
C: USER blahblah@client1.com
S: +OK, send pass
C: PASS mypassword
S: +OK 0 message(s)
C: UIDL
S: +OK
S: .
C: QUIT
S: +OK bye-bye
CommuniGate® Pro Guide. Copyright © 1998-2006, Stalker Software, Inc.
