We are experiencing the same issue as we were a while back with a mail file not opening.
Upon looking at console I can see the below in regards to the mail file.
Started getting "LkMgr BEGIN Long Held Lock Dump" relating to her mail file.
I have had the issue before and while I have no idea of the cause I was able to fix by completely recreating the person doc and mail file (i had tried recreating just the mail file first and while worked at first it soon had same error on new mail file).
I now have the same issue with a new user.
Started on her original file. Created a new mail file, after a few days same issue.
Recreated whole user. And again after a day or so I have gotten issue again.