A new user had been set up at Somo. When users started to send Emails to the address, they informed me that there were bounce backs. (Ticket No. 7217) The error in the bounce back was:
'The partner did not specify the domain to relay the emails.'
Therefore, I looked in to this issue and the problem was an Email routing issue. The Email routing options can be found on the users' profile screen in the Admin Centre. Once here, go to 'Account':
Under account, there are multiple options. The Email Routing settings are highlighted below:
The user who had the issues had the following settings in that section:
As highlighted above, the 'G Suite email' routing option was unchecked. So, I looked in to other accounts on the tenant and this option was ticked.
Therefore, the option was ticked on the account which was experiencing the problem and the changes were saved:
Following this, I ensured the user signed out of their account in their web browser. The browser was then closed and reopened. I then signed in to the account again. Two test Emails had been delivered.