There is no provision to use another port than 25 for this module.
25 is actually intended for MTA / MTA communication.
587 is designated for MUA / MTA submission.
See RFC-4409. Quoting:
Abstract
This memo splits message submission from message relay, allowing each
service to operate according to its own rules (for security, policy,
etc.), and specifies what actions are to be taken by a submission
server.
Message relay and final delivery are unaffected, and continue to use
SMTP over port 25.
When conforming to this document, message submission uses the
protocol specified here, normally over port 587.
This separation of function offers a number of benefits, including
the ability to apply specific security or policy requirements.