It seems email will not be delivered to some specific addresses, although it would work for others [please see attachment]. The destination emails definitely exist and I can reach them via a different system (e.g. Gmail). Doing a little digging, I found reports that this occurs sometimes when a mailbox is transferred between users (as it was the case, as per my previous posts, above), however, I could not find a solution for the problem. How can this be fixed?
From what I read, I believe it may require some action server side in connection of cache flushing but I'm not very sure.
I believe creating a new user would solve the problem, but, of course, it would be disruptive to change the email address at this point.
Help much appreciated as it's been challenging to circumvect this problem to reach out to clients with the affected email addresses.
Thanks in advance.