Argument Processing and Address Parsing

When sendmail receives input, it collects recipient names (either from the command line or from the SMTP protocol) and generates two files. One is an envelope that contains the header and a list of recipients. The other file contains the body of the message. The sendmail program expands aliases, including mailing lists, and validates as much as possible of the remote recipient. Then sendmail checks syntax and verifies local recipients. Detailed checking of host names is deferred until delivery. As sendmail verifies local recipients, messages are forwarded to them.

After parsing the recipient lists, sendmail appends each name to both the envelope and the header of the message. When a name is aliased or forwarded, it retains the old name in the list and sets a flag to tell the delivery phase to ignore this recipient. The lists are kept free from duplicates, preventing alias loops and duplicate messages delivered to the same recipient, which can occur if a recipient is in two different alias groups.

Users may receive duplicate copies of the same message when alias lists contain email addresses for the same person using different syntax. The sendmail program cannot always match the email addresses as duplicates of one another.