My son had a similar problem - perhaps the same fix as his can help you.
If you don't put a Micro SD memory card in the handset, everything has to fit into the relatively limited amount of installed memory, including any pictures you take ot recordings you make. When you have an SD card, it automatically goes there, freeing up enough base memory to keep your old e-mail from deleting.
In the meantime, if you install and use the Desktop Manager, you can reconcile your e-mail with Outlook, synch your calendar and contacts and memos and notes, manage your applications and make restorable backups of everything on the phone.
Thanks for investigating your alternatives.