Running an email service provider is a rough business:
a) Emails hitting inboxes are basically indistinguishable from cash at scale, and therefore you have all the fraud problems of a payments business but without a lot of the built-in fraud resolution systems
b) There exists an oligopoly of inbox providers who have a Nuke Your Business From Orbit button available to them, and that button can and will be pushed by a cron job if you do not keep your customers squeaky clean
c) The customer population is frequently low-sophistication, like the owners of flower shops or virtually anyone in the cryptocurrency economy (my apologies to professional journalists like, err, Two Bit Idiot)
d) Per-account values are, at the low end, really, really low by the standards of B2B SaaS, and as a result your customer service has to get operated in a very scalable fashion, and that implies some tradeoffs where not everybody is going to be happy with it 100% of the time.