For some reason I had thought you said 10.5, you are on 10.4.
I found the definitive answer now:
1. Open Mail.
2. Choose Preferences from the Mail menu.
3. You have an existing .Mac account so select it and click the minus button. This will not delete any of your messages from the email server. Make sure the Account Type of the account you're about to delete does not say POP; if it does then skip to step 5.
4. Add a new IMAP account with the following settings.
In the email address field, enter the primary email address you want to use to send email from, eitherXXX@XXXXXX.XXX orXXX@XXXXXX.XXX. Note: If you were not previously a .Mac member, you will be unable to send email fromXXX@XXXXXX.XXX.
- Incoming Mail Server: mail.me.com
- Outgoing Mail Server: smtp.me.com (make sure to use authentication with your outgoing mail server)
After creating the account, you can add additional addresses in the Email Address field in the preferences for your account. Separate additional addresses with a comma. For example,XXX@XXXXXX.XXX,XXX@XXXXXX.XXX,XXX@XXXXXX.XXX. When configured this way, Mail will not use aliases from MobileMe, and additional email addresses added in Mail will not show up in Webmail.
This is directly from Apple, so it should work perfectly. Again, if not let me know.
If this fixes your problem, please kindly select * ACCEPT * so I can be rewarded for my efforts.