e-mail with attachment won't sync

[HTC.Cingular 8125 user]

I have a Cingular 8125 (HTC Wizard) which is configured to sync e-mail with a Microsoft Exchange server. Mail is sent correctly, and I am able to receive and forward e-mails with attachments. BUT, when I create an e-mail with attachment on my device, it won't sync. I receive a message saying to delete it from the Outbox and try again. Of course, this is unsuccessful.

Any ideas on what is causing this?