You are already dealing with the most probable cause. Delete Outlook Auto-Complete records and add the new address as a Contact.

Redirecting


NK2Edit: Edit AutoComplete files (.NK2) of Microsoft Outlook

Are the old and new domains hosted by the same company? I'm thinking problems with the MX records. But that should affect any mail going thru the provider.

From way out in left field: Have a client, who is still sending to the old domain, do a MX Lookup on both the old and new domains.
https://mxtoolbox.com/

Thanks, Hornblower, for the feedback and suggestions.

I honestly have no idea as that was handled by someone else, but I just sent the question to our IT consultant, along with the results I got when I used the MX lookup on our current domain. It says:

SMTP Reverse DNS Mismatch Reverse DNS does not contain the hostname Information More Info
Status Warning SMTP Banner Check Reverse DNS does not match SMTP Banner

Hopefully, if that is an issue, or THE issue, he can look into it for us.

Cheers,

Ann

>> along with the results I got when I used the MX lookup

More useful to your IT people would be the MX Lookup results from one of the clients who says their mail to you is not being delivered to the correct domain.

I see. I will suggest that in the morning if they haven't gotten any further.

With one person, we tried his emailing to one of the set-up aliases for our coworker. For example, our standard emails are first initial, middle initial, and last name, though there is a workable one with the first name only before the @. This did get the email through from the outside person to the inside person, though when inside person replied, naturally it came from his main email, and then outside person couldn't just hit reply and get through. The inside person would have to remember to customize the Reply-To every time, unless there is a way to fiddle with the Outlook Rules to do that for him? I haven't sussed a way to make that happen.

With another sender, their IT is now in touch with our IT, so maybe we'll reach a solution. It's been a head-scratcher! Why just these few people have the issue, I don't know.