2 IMAP emails populating same sent folder

We have a client that uses 2 domains on different mail servers. Email-A is on rackspace, Email-B’s server is unknown. This has not been a problem so far as we know enough information to configure their email. All of the employees are using Outlook 2016.

One employee now has a problem. When they send email from Email-B the email shows up in Email-A’s sent folder. In fact Email-B doesn’t even have it’s own sent folder. I reset their outlook profile, same problem.

Here’s where it gets weird. I logged in to Email-B on my own PC and when I sent a message it appeared in my personal email’s sent folder. Again, Email-B did not populate it’s own sent folder, instead sending it to the sent folder of whatever other email is configured in Outlook at the time.

At a loss I downloaded Thunderbird and plugged in both of the users emails. Email-B once again populated without a sent folder. However, when I sent a message it created a sent folder within Email-B and placed the sent email inside of the correct folder. When I refreshed Outlook, a sent folder appeared with the sent message from Thunderbird inside of it.

Now if i send a message from Email-B in Outlook it still appears in Email-A’s sent folder. But if I send an email from Email-B in Thunderbird it appears in Email-B’s sent box in Thunderbird and Outlook.

I am at a complete loss.