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I am looking at this Q&A: How to open a port?.

The question title is about opening ports in general. The question "body" and the answer focus on port 25 and how to list open ports, but do not address the title question in general.

I keep coming back to this thread whenever i forget the command on how to open a port on a raspberry/debian, but do not find the desired answer even though the title indicates so. And with 61k views i suppose there might be others whose interest is not restricted to port 25.

There might be two solutions:

  1. modifying the question title (+ open new question?)
  2. place a second answer on how to open ports in general

My thoughts:

I initially went for an answer: https://raspberrypi.stackexchange.com/a/111330/115531. But then i thought i better ask here on meta.

  1. Modifying the title would have the advantage to have a cleaner question - answer fit. Downside would be that user searching for that question ("How to open port?") would still not get an answer that fast.

  2. The question seems to be very easy to find via search engines (60k+ views). One could open another question + answer, to address the general question on how to open a port and hope it will become one of the top results?

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Your answer looks fine. Even if it's unlikely to help the OP, it does answer the literal question, so people having the same question are likely to find it and benefit from your answer.

A more pedantic approach would be to ask a new question (specifically about a firewall) and self-answer it. Modifying the original question should be done with caution, taking extra care to preserve its original intent and not to make any of the existing answers invalid. In this particular case, there's only one answer (besides yours) which addresses specifically SMTP, so restricting the question to port 25 only is viable.

In general, providing a different answer to a question which already has an accepted answer is OK.

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This is a good example of why one should be careful when considering altering a question. Generalising the question and diminishing the port 25 bit would be a mistake here as it is really the most important part of the question.

Steve hints at why in his answer. While there are zillions of places one can read about how to open a port (not a criticism -- your answer is fine IMO, just not the canonically correct one), the fact that almost certainly most ISPs do block outgoing access to port 25 is something that isn't documented well online, including by the ISPs (you will probably only find some reference to it if the ISP is large enough and has a long running user forum).

Port 25 is commonly used as an open relay for email. It's unencrypted and pretty much no one, email provider wise, uses it anymore for end user connections (ie., for you to send mail through), which is why most people will never notice that they can't connect to it from home.

If you've read stats about how 95% of the world's email is junk, hopefully mostly filtered out by your email provider, it's open SMTP relays creating a port 25 based pipeline that make that possible. The reason ISPs block is because if they do not, they will suddenly find they have clients that max out sending junk through open relays (it has to come from somewhere).

[In case that's not clear, it is specifically about you sending, not getting, junk mail. Put another way, sending mail through 25 pretty much screams "spammer".]

As far as I am aware this is the only port this applies to, which also makes it a bit confusing; if an ISP doesn't block anything else, which I think probably most of them do not on this level, then it is easy to presume they don't do it at all, especially if you think it's something normally used by email providers for end user clients.

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