Recently had a customer use crazydomains for their Emails. Incoming email stopped with the error below
550 Please turn on SMTP Authentication in your mail client. mail-wm1-f50.google.com [188.8.131.52]:38049 is not permitted to relay through this server without authentication.
I check the MX / NS servers to make sure they were pointing a valid records.
Called support and they asked me to remove off Automatically Detect and change to Local Mail Exchanger , it took 15 minutes to apply and starting working again. Guessing their internal DNS had an issue