If you send email to an internal account which hasn't been set up yet, Mailjet decides that it is an attempt to send spam and "preblocks" the address for 90 days. This doesn't make sense for internal accounts.

For example, suppose I have a new employee named Susan and I add susan@mydomain.com to a mailing list, or I send her mail through a system that uses the Mailjet API (which is the case that happened to me). But, oops, the person setting up the email account hadn't set it up yet! Mailjet now "preblocks" her address for three months and, worse, it does so almost invisibly — there's no indication this has happened unless you happen to go to the Mailjet page that shows what's been delivered. So Susan doesn't get some of her email and nobody knows why. To fix it, we have to either change Susan's email address (not practical, business cards have been printed, it's already been given out to many people, and there are even worse cases than that) or we have to submit a request for somebody at Mailjet to fix it manually.

But the whole problem is avoidable.

Mailjet knows that we own mydomain.com — we've set up a mailjet_domainkeys record and multiple SPF records for mailjet in our DNS server. Given this, Mailjet should not "preblock" email that we are sending to our own domain. That just doesn't make sense. And, even better, it should forward the "bounce" message back to the sender address rather than having the message just vanish.