Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 371)

Mail Service 371
Internet Message Access Protocol (IMAP)
Internet Message Access Protocol (IMAP) is the solution for people who need to receive mail
from more than one computer. IMAP is a client-server mail protocol that allows users to
access their mail from anywhere on the Internet. Users can send and read mail with a
number of IMAP-compliant email clients.
With IMAP, client users’ mail is stored in a remote mailbox on the server; mail appears to
users just as if it were on the local computer. IMAP delivers mail to the server, as with POP,
but the mail is not removed from the server until the user deletes it.
IMAP follows the typical client-server model. The user’s computer can ask the server for
message headers, ask for the bodies of specified messages, or search for messages that meet
certain criteria. These messages are downloaded as the user opens them.
Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP) is a protocol that is used to send and transfer mail.
Since SMTP’s ability to queue incoming messages is limited, it is usually used only to send
mail, while POP or IMAP is used to receive mail.
SMTP Alternatives: Sendmail and Postfix
Instead of the SMTP mail service of Mac OS X Server, you can use another mail transfer
agent (MTA), such as the UNIX programs Sendmail and Postfix. If you choose to use another
mail transfer agent, it handles all incoming and outgoing SMTP mail. In this case, mail sent to
local email users is delivered to the other mail transfer agent. Then Mac OS X Server transfers
incoming mail from the other mail transfer agent for final delivery to email users using the
POP and IMAP protocols. POP and IMAP continue to function as usual, but SMTP mail is now
subject to the rules and settings of the other mail transfer agent.
The UNIX Sendmail program is included with Mac OS X Server and is configured to work
correctly with Mac OS X Server mail service. To use Sendmail, you must set Mac OS X Server
mail service to use an alternate mail transfer agent and you must start Sendmail. For more
information about Sendmail, see this Web site:
www.sendmail.org
If you want to use the Postfix program instead of Sendmail, you must install and configure
Postfix. Then you must set Mac OS X Server mail service to use an alternate mail transfer
agent and you must start Postfix. For more information about Postfix, see this Web site:
www.postfix.org