ICloud+ Custom Mail Domain - "user does not exist"
Using ICloud+ and set up my custom mail domain with 3 emails. two of the emails work fine and get sent through to my apple mailbox, but my third email always returns a "550 5.1.1 User Does Not Exist" error.
Tried removing the email and adding it again, also tried removing the whole custom mail domain and setting it back up again (including DNS records etc).
The only difference is the one that doesn't work is only two digits long (jo@mydomain.com) and the others are a bit longer (e.g. hank@mydomain.com) and also that this email address is also the one used for my apple ID; but can't think why either of those would stop it working.
Any ideas? I'm completely stumped. I've already phoned iCloud+ support who went around the houses, watched me setting it up via screen sharing, then when it was blatantly an apple issue (as the apple mailserver is the one returning that the user doesn't exist error) they just palmed me off with "Speak to your hosting provider" :P they saw me input the DNS settings, they are fine, if they were not the other two wouldn't work. *huff*
Any suggestions welcome! Cheers in advance.