smtpmail-sendto-domain

smtpmail-sendto-domain is a variable defined in `smtpmail.el'.
Its value is

nil

Documentation:
Local domain name without a host name.
This is appended (with an @-sign) to any specified recipients which do
not include an @-sign, so that each RCPT TO address is fully qualified.
(Some configurations of sendmail require this.)

Don't bother to set this unless you have get an error like:
Sending failed; 501 : recipient address must contain a domain.

You can customize this variable.