Mail is not downloading older emails

I have 3 email accounts on my ipod. 2 of them belong to optimum, the third is a yahoo account. When I look for older mail, I can go back to the beginning of the account on 1 optimum, and on yahoo. But the other optimum email seems to auto-delete from my account 50 separate messages (not including those grouped together). I am missing emails that I've flagged for followup. There is no option for "load more messages", as there is on the other 2 accounts. Of course, it is my most important email, and very frustrating...

I have tried looking at it through "Inboxes" as well as "Accounts". The one that deletes still shows all emails sent since I've owned the ipod, old drafts, etc. I have checked the ipod settings against the other 2 email accounts, and there are no differences. I've connected to the computer and checked through itunes, as well as through the optimum site, and can't see anything that looks like it would change this. The ipod is not synced with Outlook.

When I log into the account from a browser, it shows all the emails missing from my ipod, still in the inbox. Can someone please advise how to get my emails back?

Thanks in advance.