Hi Charlie, thank you for returning to the thread to keep us updated!
If there are any other customers who are also experiencing this issue we would urge them to join this thread as we can only raise this issue on a case by case basis. If this is a more widespread issue the more examples of this we have the easier it is to raise internally.
Can I ask a few extra things just to help us potentially diagnose the issue:
-Have you ever received the emails from this mailer list?
-Do you know if the mail has a large number of recipients? (This could include CC or BCC recipients)
-Have you tried saving the email address sender in your contacts?
-Have you recently run an anti-virus scan on all devices associated to your email account?
Let us know and we will do our best to offer further support!
All the best.