I was curious so I tried to register with a Proton Pass alias which is @passmail.net and they also refuse those. I think they are afraid of services which allow to easily create multiple aliases, because people could create multiple accounts very easily (scammers maybe)? It’s a dumb rationale because it’s not much harder to create many Gmail accounts but that’s the only reason I can think of.
Maybe you’re using light roasted coffee, which usually requires a higher quality grinder to extract properly. You could switch to something darker which for sure will reduce the acidity levels, or get a cheap quality hand grinder. I’ve had a 1zpresso for a while which does the job, but I finally got a good electric one too because it’s only fun for a few weeks to grind by hand for espresso (it takes like 45 seconds)