usually isIt's possible there is a reason...
As I already shared, years ago when my business email was briefly black listed it was because my web host was sharing mail servers between multiple customers. Apparently the other customer using my email server was either hacked, or a bad actor.Like your mailserver's ip being used by spammers. Either they register new accounts all the time, paid with stolen CCs, or your provider's mailserver isn't very secure.
but it may not be your faultIt rarely happens for no reason.
I have been using the same web host for years and they are pretty good in my experience, despite that one time email hassle (years ago). "1 and 1" IIRC is a German based company, they have since changed their name in the US market to "IONOS". I have no qualms about recommending them for general web hosting et al.Using a reliable mail/web provider and a different company as domain registrar and DNS provider helps a lot. Only, it's hard to tell which are the reliable ones. If you only need mail, look at Protonmail, or Tutanotta. Both are technically excellent. Much better than gmail, or, the worst of all of them, Hotmail.
There is no free lunch, free email means you are the product....Obviously, a 1$ a month provider like one.com isn't going to worry about your email. Mind you, one.com has a terrible offering for that price. Just don't expect them to be equal to a provider like mine, who doesn't advertise, but costs at least 50$ a month.
JR