Reply By Email not working for specific user

My forum ( 3.1.0.beta2) has Reply By Email enabled, and this is otherwise working for other users.

However one user, who is receiving forum posts by email is finding that his emailed replies to these posts never get through to the forum.

He sent me a copy of one such reply, and I can’t see anything obviously wrong with it: the ‘From’ address is the one that the user has nominated for his forum account, and the ‘To’ address is in what looks like the correct format for a reply by email:

However I can’t find any record of any emails being received from this sender in the forum logs., and the associated post has not appeared.

What might be going on here?

He is using a gmail address - could that be relevant?