Been there, done that. Two years ago Mailchimp deactivated our account without notice, and no explanation at first. This also impacted our transactional mails via Mandrill as the account was linked to the deactivated Mailchimp account.
Even after a lot of pleading they didn't reactivate it, and there responses were generally unhelpful and in hostile language.
In the end it turned out that appart from our paid(!) main account, an intern from the marketing department created another account some time ago which hadn't been in use for a few months already.
So in the end a coworker and me spent the night rewriting our transactional mailing to hook up Sendgrid instead, with no problems since then.
Did you ever consider SES? I've been using Sendgrid for about two years to handle lead distribution to client CRMs (Automotive). They have good documentation and are super easy to integrate. Although, I wish the delivery notification endpoint supported multiple webhooks.
I don't think we really considered SES, to be honest.
We had to move pretty fast when we made the switch. My coworker tried Mailgun (which didn't verify our domain fast enough), while I tried Sendgrid.
There was also the added bonus that we had a free paid tier from Sendgrid as part of our accalerator program, so we had planned to switch to Sendgrid anyway.
In the end it turned out that appart from our paid(!) main account, an intern from the marketing department created another account some time ago which hadn't been in use for a few months already.
So in the end a coworker and me spent the night rewriting our transactional mailing to hook up Sendgrid instead, with no problems since then.
Fuck MailChimp + Mandrill!