15-20 and maybe 5-10 spams

That explains. Mine is around 1m+ daily and FastMail sucks at this volume.

> 1m+

Is that one thousand plus? (Certainly not one million plus I hope.)

Either amount is pretty bad, although I suppose if you are a well-known person that people are trying to reach, that would make sense.

If not, and it's a bunch of newsletters or spam, I think you should try to make better email choices. (Be more discretionary with how you give out or publish your email address(es).)