All my mail in my inbox deletes by itself

Hello Heidi.

Are you accessing your .Mac account with another email client on the same Mac or with an email client on another computer?

You're welcome.

Are you using another email client like the Mail.app (Entourage, etc.) on your Mac to access the same account or using an email client on another computer to access the same account?

Are you using any Rules in Mail especially any Rules to sort received messages by category to user created "On My Mac" mailboxes?

Have you changed your .Mac account from a .Mac type to a POP type account?

A .Mac type account behaves as and is really an IMAP type account so instead of messages downloaded from the server (as with a POP type account), the received or Inbox messages in Mail are synchronized with what is available at the server instead.

Try using the Rebuild Mailbox function on the account's Inbox mailbox. Open "In" in the mailboxes drawer by selecting the black triangle besides "In" pointing it down.

Select the Inbox mailbox for your .Mac account and at the menu bar, go to Mailbox and select Rebuild.