I'm running 4 email servers, the oldest is from 2011 or so, to this day.
4 different countries, different AS.
I'm always puzzled by all the issues described by others. I had about 4 issues during these 15 years, all 4 were solved by reading the bounce message and doing what was asked in the message (contacting support of the destination).
Same. People just remember it's hard because they don't test, don't like responsibility much and esp. because they test this in production not on a spare domain. Then all you remember from it is the 10 people shouting at you and that you need to fix it NOW and it's all super stressful so they never attempt this again.
I had a Digital Ocean VPS and it was fine about 7 years but on year 8 the entire /24 got blacklisted and it was game over
Took about 6 months initially of fiddling to get other email services to accept mail DKIM, DMARC, SPF, PTR, web form requests
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