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Is it necessary to allow incoming connections to port 25 to send (not receive) mail?

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nope. outgoing to 25 are necessary for SMTP (sending email). incoming to 25 are necessary only if you have mail server on your machine. – mnmnc Apr 12 '13 at 10:14
@mnmnc What if I have an exchange server and I want to send email from that Exchange server to another domain? – Fide Rogers Apr 12 '13 at 10:16
if you want to send emails from your own server and you want to use SMTP protocol then port 25 should be opened. But if you are sending only localy to your server - so you log in to server and send email from server's local account - then you should open the port 25 only on interface so it will not be accessible from outside of the server. If You are sending emails from other computer via this Exchange server using SMTP - then you must open port 25 on interface with external IP address - for example eth0. – mnmnc Apr 12 '13 at 10:22
@mnmnc It's the first case you mentioned, I want to send emails from my server using Exchange SMTP. So to clarify, do I need to open port 25 incoming on that server or will port 25 outgoing suffice? – Fide Rogers Apr 12 '13 at 10:26
You need to allow incomming connections to port 25 on loopback interface to your server. In order to allow your server to send/relay this message to other mailing server, You need to allow connections to port 25 from your server - meaning outgoing to 25. – mnmnc Apr 12 '13 at 11:03
up vote 0 down vote accepted

Some mail server somewhere must receive mail sent to your domain. Provided you're not talking about that mail server and that mail server exists somewhere else, then no. Not every machine that sends mail must be capable of receiving it. However, to have a domain that operates as a legal source for email, it must have a mail server somewhere that accepts postmaster mail and bounces.

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