Outlook lost messages

Hi,

I met a problem this morning. One of the staff told me she didn’t recevie an email from a external client. I checked the Message Tracking Center and it displayed
” Message delivered locally to store to XX@domain.com”, I also checked outlook rules, junk email options, messagelabs, changed views, searched the senders in outlook but still can’t see it.
Now I don’t know what to do next.

Any suggestions will be greatly appreciated.