I use TLS for my postfix and dovecot,and I ready test centos 7 mail server by various tools,result as below:

Priority: 5 Connection established: yes TLS: yes SSL: yes Cipher: TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384 Uses secure cipher: yes

Then I block port 25 in iptables,but I cannot receive mail any more.

As I ready use TLS,which means I only use 587 or 465,why blocking port 25 cause I cannot receive mail?


The TLS is for when a client connect.

The port 25 is when a mail server send you a mail without enforcing the encryption, as such you can’t workaround the need of the port 25 if you want to receive from everyone.

A remote server will check your MX, and will send to your port 25 the mail.

You could check that question in example; https://stackoverflow.com/questions/18139102/how-to-get-around-an-isp-block-on-port-25-for-smtp

or that article; https://tipstricks.itmatrix.eu/using-tls-for-mail-delivery-from-postfix-to-another-tls-activated-mail-server/


Port 25 is required for a mail server to receive email from other email servers. Port 587 & 465 is used by mail clients (ie, Microsoft Outlook) to send email from a client to a server. In order for your server to receive mail, port 25 must be allowed.

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