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I mean I can use port 465 to submit an email(destined to sombody@gmail.com) to my Postfix server in a secure way, but which port does Postfix use to connect to gmail's smtp server? If Postfix uses port 25, is the email transported without encryption? And does gmail's smtp server accept anonymous connections from other mail servers? By the way, how to see the port postfix connects to gmail server on CentOS?
By default it should be using port 25 though that doesn't mean that it can't use STARTTLS or SMTPS to securely communicate with other servers. It depends on their and your configuration. Forcing smtp outgoing mail encryption on postfix on server fault
– Seth – 2018-03-23T11:11:36.163