Mail User Agent
The mail user agent is the program that acts as the interface between the user and mail transfer agent. The sendmail program is a mail transfer agent. The Solaris operating environment supplies the following mail user agents.
/usr/bin/mail
/usr/bin/mailx
$OPENWINHOME/bin/mailtool
/usr/dt/bin/dtmail
Mail Transfer Agent
The mail transfer agent is responsible for the routing of mail messages and the resolution of mail addresses. This agent is also known as a mail transport agent. The transfer agent for the Solaris operating environment is sendmail. The transfer agent performs these functions.
Accepts messages from the mail user agent
Resolves destination addresses
Selects a proper delivery agent to deliver the mail
Receives incoming mail from other mail transfer agents
Local Delivery Agent
A local delivery agent is a program that implements a mail delivery protocol. The following local delivery agents are provided with the Solaris operating environment.
The UUCP local delivery agent, which uses uux to deliver mail
The local delivery agent, which is mail.local in the standard Solaris release
Chapter 27, What's New With Mail Services (Reference) provides information on these related topics.
Mailers
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 "Building the sendmail.cf Configuration File (Task)" in Chapter 25, 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 /usr/lib/mail/README.
Simple Mail Transfer Protocol (SMTP) Mailers
SMTP is the standard mail protocol that is used on the Internet. This protocol defines these mailers.
smtp provides regular (historic-style) 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" and "New Delivery Agent Flags" in Chapter 27, What's New With Mail Services (Reference).
UNIX-to-UNIX Copy Program (UUCP) Mailers
If possible, avoid using UUCP. For an explanation, refer to http://www.sendmail.org/m4/uucp.html or do a search in /usr/lib/mail/README on this string: USING UUCP MAILERS.
UUCP defines these mailers.
| uucp-old | 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. |
| uucp-new | 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.
| uucp-dom | This mailer uses domain-style addresses and, basically, applies the SMTP rewriting rules. |
| uucp-uudom | Names in the $=Z class are sent to uucp-uudom. uucp-uudom and uucp-dom use the same header address format, domain-style addresses. |
Note - Because the smtp mailer modifies the UUCP mailer, always put MAILER(smtp) before MAILER(uucp) in your .mc file.
Mail Addresses
The mail address contains the name of the recipient and the system to which the mail message is delivered. When you administer a small mail system that does not use a name service, addressing mail is easy. The login names uniquely identify the users. Complexity is introduced if you are administering a mail system that has more than one system with mailboxes or has one or more domains. Additional complexity can be generated if you have a UUCP (or other) mail connection to the outside world. The information in the following sections can help you understand the parts and complexities of a mail address.