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Environment: CentOS, Postfix, Dovecot, Digital Ocean Droplet

Background: The mail server I just installed appears to send and receive normally. I tested my website on MXtoolbox and received 1 error that was related to SMTP. It was the only error that I received.

Unable to Connect to SMTP Host

Description: We attempted a connection to your mail server on port 25 but your server did not respond. More than likely there is no email server at this address or you have a misconfigured firewall or spam filtering system.

Question: Firewalld is blocking port 25. The only ports I have open are, 80/tcp 443/tcp 587/tcp 993/tcp. Do I need port 25 to be open for the mail server to function correctly and consistently or is this error unimportant?

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If you want to be able to receive mail from other mail servers (as distinct from end users/MUA's) yes, port 25 needs to be open and listening all the time, and available to accept connection from anywhere.

(If you are doing this other then experimentally, please consider having someone come alongside you - you are likely missing a lot of nuance around mail delivery)

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    Totally agree: just one of the „nuances“ is opening a relay for all the spammers in the world to send thousands if mails over that server – Torsten Link May 13 '20 at 6:58

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