I have the same issue with @Tutanota : my emails never make it to some recipients. I’ve never sent a single spam on my life. The only factor I was able to isolate is that anything going to Gmail or a Google-managed address doesn’t makes it, which I assume means it was caught in some spam filters. But it’s not the only factor. I just haven’t figured out what makes it so I can’t write to some other addresses.

It’s really a PITA, forcing me to keep another email.