Unable to send message;Reason:559 Message not committed due to internal error

Unable to send message;Reason:559 Message not committed due to internal error

I have an Account/Organisation with two users. I am able to receive email to both accounts but I am only able to send on one. 
For the second email address I am getting the following error:
Unable to send message;Reason:559 Message not committed due to internal error

This happens both in the Web Mail and using the Zoho App.

Any ideas?