10.3.2 Mailname

The mailname of a host is the name that mail-related programs use to identify the host. The file /etc/mailname contains of this name followed by a newline. The mailname is usually a fully qualified domain name that resolves to one of the host's IP addresses. See mailname(5).

What the recipient of e-mail sees in the From: header of mail sent by your Debian host depends on how Mail User Agents (MUA) and Mail Transfer Agents (MTA) are configured. Suppose a local user foo sends a mail from a host with mailname myhost.dom. The From: header of outgoing e-mail will be:

• "From: foo@myhost.dom" if the MUA has no From: header set;

• "From: bar@myhost.dom" if the MUA has "From: bar" set;

• "From: bar@bogus.dom" if the MUA has "From: bar@bogus.dom" set.

Even when the MUA has a From: header set the MTA may add a "Sender:foo@herman.dom" header to indicate its true origin.

Of course when any involved MTA performs address rewriting as discussed in Setting up a catchall for nonexistent email addresses under Exim, Section 9.6.1.3 and Configuring selective address rewriting for outgoing mail under Exim, Section 9.6.1.4, the e-mail address seen by the recipient can be changed to something else.