Hi,
if you have a critical error (a single message your site is experiencing techincal problems), as said the details of that error is sent by your site to the email set on the main settings of your site.
Did you check which email is set? Is it a real mailbox you can check?
About the initial problem, Maijet is reporting an error about a suspeneded account. Go to the mailjet service to find more about that error: https://www.mailjet.com/
Log-in and you should have a notice or the like, or open a ticket with Mailjet and report the error you’re seeing.