smtpmail-queue-mail

smtpmail-queue-mail is a variable defined in `smtpmail.el'.
Its value is

nil

Documentation:
Non-nil means mail is queued; otherwise it is sent immediately.
If queued, it is stored in the directory `smtpmail-queue-dir'
and sent with `smtpmail-send-queued-mail'.

You can customize this variable.