Tried Mandrill/Mailchimp transactional email earlier this year and it was trash, we already had an established marketing account and they insta banned me for sending a single test email to myself before we even started using it. They flagged the account for "suspicious access" with no way to resolve even though I followed all their steps.

It seems that most of these transactional email companies concept of "100% delivery guarantee" excludes you from that guarantee... they aggressively and pre-emptively protect their servers from being blacklisted by autoblocking you first based on some heuristics and then very slowly or possibly never unblocking you manually later through useless support staff. Guilty until proven innocent.

Settled on Postmark (after trying pretty much all the popular transactional mail services without success), which has so far been golden, customer support will actually help you resolve an issue if there is one, plenty of forewarning with transparency before any blocking, the front end is exemplary of clear usable UX. It also lets you sandbox transactional mail into "servers" so that one bad source can't affect another.