When sending emails through the GMS-cloud service, you may see an error:
‘542 5.7.1 Mail relay not allowed at this server’
When sending emails via the server, users are checked to ensure they are authorised to do so. Usually, An account will have logged in via POP or IMAP and so will be permitted to send email because of this.
However on some occasions the IP address of the user will change, or will be different, so this method does not work.
If you see this error repeatedly for users (rather than, say, pressing send and receive again makes it go away) then you need to enable SMTP authentication for that user in the mail client.
From the Tools menu, choose “Accounts.”
Select click the correct Mail account (e.g. mail.gms-cloud.com) and click “Properties.”
Select the “Servers” tab.
Check the box next to “My Server Requires Authentication.”
In other clients you should research the correct method for doing this for that particular client
This should then solve this problem for the user.