Mailer is a sendmail-specific term. A mailer is used by sendmail to identify a specific instance of a customized local delivery agent or a customized mail transfer agent. You need to specify at least one mailer in your sendmail.cf file. For task information, refer to Changing the sendmail Configuration in Chapter 13, Mail Services (Tasks). This section provides a brief description of two types of mailers.
For additional information about mailers, see http://www.sendmail.org/m4/readme.html or /etc/mail/cf/README.
smtp provides regular SMTP transfers to other servers.
esmtp provides extended SMTP transfers to other servers.
smtp8 provides SMTP transfers to other servers without converting 8-bit data to MIME.
dsmtp provides on-demand delivery by using the F=% mailer flag. Refer to Changes to the MAILER() Declaration From Version 8.12 of sendmail and Additional Delivery Agent Flags From Version 8.12 of sendmail.
If possible, avoid using UUCP. For an explanation, refer to http://www.sendmail.org/m4/uucp_mailers.html or do a search in /etc/mail/cf/README on this string: USING UUCP MAILERS.
UUCP defines these mailers.
Names in the $=U class are sent to uucp-old. uucp is the obsolete name for this mailer. The uucp-old mailer uses an exclamation-point address in the headers.
Names in the $=Y class are sent to uucp-new. Use this mailer when you know that the receiving UUCP mailer can manage multiple recipients in one transfer. suucp is the obsolete name for this mailer. The uucp-new mailer also uses an exclamation-point address in the headers.
If MAILER(smtp) is also specified in your configuration, two more mailers are defined.
This mailer uses domain-style addresses and, basically, applies the SMTP rewriting rules.
Names in the $=Z class are sent to uucp-uudom. uucp-uudom and uucp-dom use the same header address format, domain-style addresses.
Because the smtp mailer modifies the UUCP mailer, always put MAILER(smtp) before MAILER(uucp) in your .mc file.