Sorry, I should have said significant problems. If you send email on anything like a large-scale basis you’ll get on some kind of blacklist at some time – all you need is one user sending email to someone with a twitchy spam reporting reflex. What I was thinking about, though, is that it’s usually transient – e.g. out of hundreds of thousands of emails I’ve never had one bounce that way instead of being delayed, which is better then self-hosting on a long-time class B over a similar timeframe.