Our technical account manager informed us, that it is not possible to have custom login pages as the logn pages are global for every Mailjet customer and Mailjet is not interested to maintain custom login pages per customer.

Especially for cases where global pages are in place and you are not interested to offer customization there is an easy way of at least cusotmizing the pages just by using some dedicated settings per customer. Example:

• for us it would be already helpful to deactivate "Create new account" link
• a very helful setting would be "show only SSO Login Link", not the user passsword form
• Need Help link, custom setting: our own support mail address instead of yours

All of this functionalities can be offered just having some customer specific settings. I understand the challenge that at that time there might be no customer specific scope as the login did not happend and there is no customer scope.
BUT:
you could use some URL Parameters like https://app.mailjet.com/signin?customer=mycompany , we will than only communicate that loign page as valid login page.