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Why are different SMTP ports used at Sussex for sending email?

The worldwide standard port number for SMTP connections (for sending email) is 25,  The Exchange mail system (for staff and researchers) operates in a different way and may not require you to set any particular port number.

The recommended SMTP port number for Sussex users is 587, Port 587 is the official "Message Submission" port, as defined by Internet standard RFC 2476 and used for message submission by trusted, authenticated users.  However, some other email client programs may require port number 465.  Why so many different ports, you may ask?

The reason alternate ports are used is that some Internet Service Providers (ISPs) block or intercept email traffic destined for port 25, as a sensible anti-spam policy. It prevents viruses on their customers' home computers from emailing directly to third party mail servers. We do the same thing for most of the computers on the Sussex campus other than for known email servers and a very few exceptions.

At Sussex we prefer to use port 587, with TLS ('Transport Layer Security') encryption.  TLS (also known as SSLv3) allows negotiation of the method of encryption: your email client makes a plain text connection, then sets up the encryption. Unfortunately, some email clients don't support this method. In fact, some try to support it, but the implementation of the method may be incorrect or incomplete, and they fail.

Port 465 uses SSL ('Secure Sockets Layer'), where the email client makes an already encrypted connection. Some email clients support SSL on port 465, where they don't work with TLS. In fact, some other email client versions use TLS on any port except 465 where they use SSL.  Now because their implementation of TLS is buggy, they have to use port 465.

The Sussex system also supports port 25 with TLS, but we advise people to use one of the alternative ports because it is simpler to be consistent.

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This is question number 1359, which appears in the following categories:

Created by Andy Clews on 3 September 2007 and last updated by Gemma Sturtridge on 26 March 2020