The method below will work if you get your e-mail via POP. Generally, if you use Mail, Thunderbird, or Outlook.
1) Setup a Gmail account.
2) In that account, enable IMAP.
— Settings > Forwarding and POP/IMAP > Enable IMAP
3) In your local client, add Gmail as an IMAP account.
— For Mail in Mac OS X 10.5, the process is as follows
— a) Under Preferences, go to “Accounts” and click the “+” button.
— b) Enter your name, email address @gmail.com, and your password for that account.
— c) Be sure to uncheck “Automatically set up account” and click “Continue”.
— d) Set Account Type to IMAP and click “Continue”, click “Continue” again, and then click “Create”.
— e) You should now see Gmail as an account in your client. For other clients the process should be similar.
4) Drag all of your current mail to your Gmail account. Once all your mail appears in this account, you can remove Gmail from your accounts list. You should now have all of your current mail backed up to Gmail.
5) For future e-mail you want to Gmail to download it via POP. First, in Gmail, go to Settings > Accounts > Add another mail account
6) Enter your e-mail address and click next.
7) Fill out your password, change the POP server if necessary, and check “Leave a copy…” I also suggest labeling and archiving the incoming messages.
Now all your current mail should be in Gmail and all your future e-mail should appear there as well. Only thing to remember is that if you don’t log in to your Gmail account every nine months, it will be deactivated. I use my Gmail account for various sites that need an e-mail so this isn’t a problem for me.