Guide to Unix/Explanations/Mail Server Overview/Outgoing mail
Outgoing mail is first written to a spool directory on the client machine
Outgoing mail produced by the mail client is first written to a spool directory.
Mail Submission Agent
The mail submission agent takes mail from the spool directory and forwards this to the listening mail transport agent on the mail server via SMTP.
The mail transport agent forwards the mail to the internet service providers SMTP server for delivery or delivers the mail directly if the recipient is local.
A copy is placed in the sent items folder
The mail transport agent places a copy of the mail into the users sent items folder on the mail server.