Essential System Administration, 3rd Edition

Retrieving Mail Messages

As we’ve already seen, sites that connect intermittently to the Internet can complicate mail relaying and delivery. The central issue for such sites is the method for forcing mail to be sent and retrieved periodically in some automated way. Basically, the local queue needs to be flushed (e.g., via sendmail -q or postfix flush) when the connection is made, and mail for local users needs to be retrieved. (Of course, these two processes can be handled by different servers and so need not happen at the same time). Sending local mail is easily handled by adding the appropriate command to the connection script (or creating a script that activates the connection, flushes the queue, and then terminates the connection).

Retrieving mail can be performed manually via the SMTP ETRN command on remote servers that allow SMTP connections and support the enhanced SMTP protocol. Here is an example:

# telnet kevin.ahania.com 25
Trying 10.0.19.223...
Connected to kevin.
Escape character is `^]'.
220 kevin.ahania.com ESMTP Sendmail 8.11.0/8.11.0; 
Mon, 16 Apr 2001 11:22:54 -0400
EHLO astarte
250 kevin.ahania.com Hello astarte
...
ETRN mailhost.zoas.org

The final command requests mail for the specified host.

The fetchmail program, written by Eric Raymond, provides automated mail retrieval capabilities. It is a powerful program that supports a variety of transport protocols and authentication mechanisms. It operates by retrieving messages from a remote mail server and sending them ...