Some SMTP servers have special requirements. The following variables implement support for common requirements.
smtpmail-local-domaincontrols the hostname sent in the first
HELOcommand sent to the server. It should be set only if the
system-namefunction returns a name that isn't accepted by the server. Do not set this variable unless your server complains.
smtpmail-sendto-domainmakes the SMTP library add ‘@’ and the specified value to recipients specified in the message when they are sent using the
RCPT TOcommand. Some configurations of sendmail requires this behavior. Don't bother to set this unless you have get an error like:
Sending failed; SMTP protocol error
when sending mail, and the debug buffer (see Debugging)) contains an error such as:
RCPT TO: someone 501 someone: recipient address must contain a domain