>>Usually, ISP provide their own SMTP server when they block port 25.
>>Also, if you have business account, most likely you can ask them to unblock it
>
>I gave up on using that SMTP test server that uses port 25. Tried again with my Exchange server and got farther, but had authentication issues. Next, I'm going to try with one of my various other email addresses that may make things simpler.

Reading this thread I had a dozen deja vus. Had the same problem back in 2000, as even then spam was already a problem and many SMTP servers didn't want to get involved, specially didn't want to have to invest in more capacity only to serve as spammers' unpaid accomplices. So back then sending emails from an app was becoming a problem (which, luckily, we didn't really have to solve then, that bit of the project was abandoned soon).

I see that over the years it didn't get any better. Last time I remember, with Exchange* server involved, the only solution was to send from an email on that server, with in-house authentication. Everything else was, if not impossible, not worth the effort.

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* not every exchange server, only microsoftian exchange...