But then Vultr.com is not un-blocking port 25 by request anymore.

That's why I had to switch to a SMTP service.

But the reader should be aware that these writeups of how to do X often involve the writer/publisher getting referral kickbacks from the commercial service they're describing.

I'm about to be in a position of doing something like those writeups, as a microstartup, and I'm not entirely comfortable with the affiliate programs. But the companies monetizing with privacy-invading ubiquitous profiling trackers (sometimes euphemistically called "showing ads" and "analytics"), and otherwise selling personal data, have spoiled most potential willingness of readers to pay for content. So, affiliate programs with an obvious potential conflict of interest is the only way I've thought of to fund the work.

Yes, I get a lot of email. But it's almost all transactional or subscription. The number of emails I send or receive with other humans is pretty dang low. Most institutions these days require using their platform for communications. Most people I care about who I communicate with electronically I do over SMS or Signal or occasionally a Mastodon message.

I still own the domain, so I could easily pick up up and move to a different mail service in probably just several minutes of setting up an account and changing some DNS values. So while not fully independent, the time spent getting outbound email right is going to have less impact than other changes I could make.

Spam pretty much destroyed e-mail as an actually open protocol. Spam destroys all open systems.