The one time I sent out a mass email using another server and it arrived in my spam folder, Fastmail Customer Service gave me instructions which would allow them to look at the email and help me figure out what the issue was, but I decided not to bother and didn't use that server again.
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).)
Recently they let through a phishing attempt against their own service. It was clearly not from them (to me) and didn't have a green dot or whatever. I was surprised and a bit disappointed that I had to see it at all.
I like their service.
How so?