Intermittently emails sent from Windows Live client come back into the POP inbox

Intermittently emails sent from Windows Live client come back into the POP inbox

We have a user on zoho with a bizarre intermittent problem.
The user predominately uses a desktop client, Windows Live Mail.

The user has her email account set up with both POP and IMAP (the later mainly to review the spam folder due to frequently miscategorised spam and as a backup to the POP). She sends emails through zoho SMTP authenticated.

She mainly uses the POP acount.
Sometimes (about 1/5th of the time) when she forwards an email to someone other than herself (from either the IMAP or the POP folders), the email comes back through POP after a few minutes to her POP inbox.
Note: The email does NOT show in the IMAP inbox
I cannot see any pattern to this.

I have looked at two very similar emails forwarded from/to the same recipient. One ended up in the POP inbox and one did not. The headers look almost identical other than the time, id and subject.

This is quite annoying and confusing for her.

Has anyone else experienced anything similar with zoho?

Are there any fixes?

Thanks