Email port 587

Port 587 is a special TCP port reserved for authenticated submission of email messages. Authentication can be done over the standard port 25, but 587 has the advantage that it is seldom blocked by networks providing Internet access to travelers. The standard email port 25 is frequently blocked due to widespread abuse of these networks. This abuse does not occur with port 587, because servers listening on this port require authentication.

RFC-5068 BCP-134 Hutzler, et. al., "Email Submission Operations: Access and Accountability Requirements", 2007.

RFC-4409 Gellens, Klensin, "Message Submission for Mail", 2006.