Message Delivery

The send queue is grouped by the receiving host before transmission to implement message batching. An argument list is built as the scan proceeds. Mail being sent to files is detected during the scan of the send list.

After a connection is established, sendmail makes the per-mailer changes to the header and sends the result to the mailer. If any mail is rejected by the mailer, a flag is set to invoke the return-to-sender function after all delivery is complete.

The sendmail program sends the message to the mailer using one of the same interfaces used to submit a message to sendmail (using the conventional UNIX argument vector or return status, communicating over a pair of UNIX pipes, or using SMTP over a TCP connection). Each copy of the message has a customized header.