Help Emails Deleting Themselves

[smp351]

I have a client who switched from Blackberry to the Droid Pro. He is on a regular POP3 mail server. He likes having all his email stay on his phone if possible. The blackberry would do that. It seems with the Pro, it occasionally deletes messages from his phone. I just checked and the inbox has a ton of email but it goes only back to Oct 27th. Is there an internal storage limit that is being reached and that is why emails are being deleted? Anyway to switch it to the SD Card? I tried giving him K-9 email but he did not like it. Also his sent items are no longer being shown in the sent folder. They just stopped a few months ago. Any help would be greatly appreciated in this matter. Thank you.