I have a bit of experience with running email servers. I can't really say that I had similar encounters. In my experience if you get blocked by big mail provid

I have a bit of experience with running email servers. I can't really say that I had similar encounters. In my experience if you get blocked by big mail providers it's almost always due to some reason. What's tricky is that it may be hard to tell what exactly is wrong, because they won't necessarily tell you (or not in an easy way). Some advice what I'd do to try to find out what's going on: 1. Take a sent example mail that is like the blocked one (but obviously one that reached its target destination) with all headers and run it through spamassassin. Don't just look if it hit the spam score (then you did something terribly wrong), look at each individual rule that spamassassin hit. They might give you a clue. A proper mail usually shouldn't hit any or very few positive spamassassin rules. 2. Check your IP at a service like valli where you can query multiple DNS black lists. If it is on any blacklist try to find out how you can be delisted. There are some rogue blacklists that make it impossible to be delisted at all, you may ignore them (google for them, their behavior is well documented), but these shouldn't be more than 1 or 2. As already said by other commenters, don't forget IPv6. 3. Read whatever error message you can get your hands on. If you're blocked on the SMTP level read the error message. If your message got sorted into a spam folder look at all the headers. If the provider blocking you has some online docs about their spam filtering read that. If they have some sort of service for mail ISPs where you can sign up to get warnings sign up there. Of course also the obvious stuff. If you do anything that is mass mailing you are in extra danger. Make sure that you allow people to unsubscribe easily, don't ignore manual attempts by them to unsubscribe ("I want to get off this mailing list") and delete invalid mail addresses.