I noticed this too, but it seems to have dropped off. In the month or so after I migrated from Google to Zoho I had a ton of spam, and was considering other options. But since then I haven't really gotten any more than I did with Google, maybe one obvious spam email every few weeks.

I would mark spam as spam during that initial period when there was a lot, so maybe there's just a learning process for new accounts as to what's actually spam?