Uhh, could you give some more information?

Are you trying to keep more messages in your e-mail client (e.g. Thunderbird) or are you trying to increase it via a e-mail provider such as Gmail or Outlook?

This is something you need to do manually on your e-mail provider. Instead of deleting mail, move a mail to a new folder. You could call it Read Mail. You can then save the mail for as long as you wish. (Note that there may be a limit on how much storage you have.) If you want, you might be able to move the folder to your hard drive.

WARNING ! ! Most e-mail providers will delete anything in the trash can after a given period or time. Check with the provider for more info.