Score:0

Is port 25 enough for running a mail server?

ye flag

A simple question, is it enough to open port 25 to internet for running a mail server that only send and recieves email, and don't have any external "clients" (imap,pop3)?

(all clients are internal)

Score:1
za flag

The server which originates mail should be allowed to make outgoing connections to TCP port 25 on arbitrary remote IP addresses. The server which receives mail should accept incoming TCP connections to port 25. Receiving server also sometimes originates mail, for instance, delivery notifications, so almost always it should be considered originating server too. Also, in both cases DNS should be allowed, which is UDP and TCP port 53 towards some DNS recursor.

Depending on the additional software you use (spam filters, antivirus, etc.), there may be additional requirements. For example, some spam filtering services use REST protocol over HTTPS, and virus database updates do the similar thing, so server might need to be allowed to connect to remote TCP port 443 of some defined IP addresses set.

ye flag
but that's outgoing, incoming ports are only 25 if I'm not mistaken?
Nikita Kipriyanov avatar
za flag
The only port where any incoming connections are accepted is tcp 25 if you keep all the processing local.
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