For configurations using Microsoft Office 365, the Fluid Topics mail server can sometimes display the following message:
SMTP error from remote mail server after pipelined sending data block: 550 5.7.750 Service unavailable.
Client blocked from sending from unregistered domains.
For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653
In this case, the owner of the "From" address must read the suggested documentation and update their Office 365 configuration.