Outgoing mail

Being a mail router, sendmail does not provide a user interface for composing and sending mail. These tasks are handled by a Mail User Agent (MUA). The standard MUA on SCO systems is the mail(C) program. Others include scomail(XC) and SCO Shell Mail.

With sendmail configured and using one of the standard SCO MUAs, whether mail, scomail, scosh mail:

1. The user composes a piece of mail with an MUA.

2. The MUA uses execmail to transfer the mail to sendmail. execmail executes a new instance of sendmail and passes any command-line flags and options it received from the MUA on to sendmail These flags and options, along with the contents of the /usr/lib/sendmail.cf file, control the operations of this instance of sendmail.

3. sendmail collects the recipient address(es) prior to collecting the entire message and parses the address(es) to determine the appropriate mailer and any aliases, forwarding addresses, or expanded address lists.

4. sendmail collects the message.

5. sendmail either stores the message in the queue for delivery later or calls upon the appropriate mailer to deliver the message now.

If additional MUAs are added to the system, they may request mail routing from sendmail by a new instance of sendmail directly without going through execmail, or using SMTP directly to the local host.