Simple Email Transfer Protocol - MythsNov 072020
There are plenty of myths out there in the internet including that SMTP can work on different ports than port 25.
While this is true (as well as for every protocol), port 25 is the reserved one for SMTP and the only one official.
What does that mean ?
Well, you can send an email to an API or all kind of web applications out there, but in fact these services are mainly just "workers"*. These workers translate what you sent to them and they must have port 25 for the SMTP protocol or ports 465/587 for SMTPS (the secure version of SMTP) opened. And here's why.
Workers work on every port the application programmer has decided them to (like 2525 or 8025 for connecting to your SMTPBOX), but that does not mean you reached your recipient yet. If you have received successful message for sending the email, you actually sent it to the worker.
After the worker has processed your data, he must have port 25 opened himself to send your email.
If you want to receive the response from the worker (if message was truly delivered or bounced), for SMTPBOXES there is thing called Feedback Response API (FRAPI) where you can instruct the worker to send a post/get request to your website.
How does that affect me ?
If you have Internet Service Provider (ISP) or web hosting that blocks these 3 ports mentioned earlier, you won't be able to send even a single letter to pretty much all of the emails in the world. Because every Email Service Provider (ESP) is listening on the official ports.
That is logical, otherwise it would be full mess with non-delivered emails as ESPs won't know themselves on which ports to communicate.
* Worker is not a real human but program code that is intended to finish some kind of task.