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I just noticed my ISP blocked both direction for port 25. Does that mean port 25 is for both sending or receiving email? Used to think it is for sending only.

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    I'm voting to close this question as off-topic because it is about the SMTP protocol, which is not Unix or Linux as defined in the help center. – Michael Homer Mar 29 '16 at 2:22
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When your machine sends mail, it may connect to port 25 on some remote server (note that most SMTP configurations accept mail on other ports as well, such as ports 465 or 587). Your ISP likely wants to allow their customers to send mail via a specific mail server (such as the ISP's own SMTP server) only.

In addition, your ISP probably wants to prevent customers running their own SMTP servers on their machines as well, which would imply receiving incoming connections on port 25.

Therefore, it makes sense for your ISP to block both directions to enforce this policy.

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