Mailers

A mailer is a sendmail specific term. You can customize a mail delivery agent. A mailer is used by sendmail to identify a specific instance of a customized mail delivery agent or a mail transfer agent.

You need to specify at least one mailer in the sendmail.cf file of all systems in your network.

The smtp mailer uses SMTP to transfer a message. SMTP is the standard mail protocol used on the Internet. This is an example of an SMTP mail header:

To: paul@phoenix.stateu.edu
From: Iggy.Ignatz@eng.acme.com

If mail is sent between two users in the same domain, the header looks like this:

To: Irving.Who@eng.acme.com
From: Iggy.Ignatz@eng.acme.com

Use SMTP for sending mail outside your domain, especially for mailboxes that you must reach through the Internet.

The uucp-old mailer uses uux to deliver messages, but it formats headers with a domain-style address, and the To: and Cc: lines are formatted by domain, much like the SMTP headers. The uucp headers look like this:

To: paul@phoenix.stateu.com
From: ignatz@eng.acme.com

Use uucp-uudom for UUCP mail to systems that can handle and resolve domain-style names. The sender also must be able to handle domain-style names and be able to receive replies from the Internet.

The uucp-old mailer uses an exclamation point address in the headers. This is one of the original mailers. The headers look like this:

To: edu!stateu!phoenix!paul
From: acme!ignatz

You can define other mail delivery agents by providing a mailer specification in the sendmail.cf file. Additional information about mailers can be found in /usr/lib/mail/README.