Mac Mail is deleting my emails

Hi all,

I bought a new iMac in mid-April and have noticed something - Mac Mail is automatically deleting emails and I can't find out where they're going.

I just got back from a week's vacation, downloaded my emails, and noticed that now the oldest email in my inbox is May 8 - four days ago.

Everything older than that is gone. The emails are not in the trash, in the deleted items, or in the spam folder. They're just gone.

The inbox seems to be the only folder affected. All my sent emails are still there. I looked in the preferences and can't find any setting that would automatically delete my emails after four days. Before I left for vacation I had emails going back to April 17.

Can anyone tell me what's going on and how to fix this? Thanks!