Whenever one user is sending any email through sophos it is getting blocked in sophos email logs as shown in below screenshot. It’s under Email >> mail logs
Whenever you are getting any blocked logs as shown in above screenshot.
Then login to sophos firewall GUI, go to email >> general settings, scroll down until you get blocked senders tab.
Then remove the concerned sender domain from the blocked senders.
Now you will be able to send email.
Hope this article helps you.