If you are using exchange and don't have Outlook you may find that the email address field in Preferences > Accounts > Account Information is disabled so you cannot add additional email addresses. You can still fix this issue in Mavericks by editing ~/Library/Mail/V2/MailData/Accounts.plist searching for the following line:
<key>EmailAddresses</key> <array> <string>firstname.lastname@example.org</string> </array>
And then adding in the address that people are actually sending email too:
<key>EmailAddresses</key> <array> <string>email@example.com</string> <string>firstname.lastname@example.org</string> </array>
Then restart Mail. If you go to Preferences > Accounts > Account Information you should now see
in the Email Address field.
I see the same problem being CC'd but in our case, we have Exchange 2010 with multiple alias's. If I set the address to the correct email address, after a few days it reverts back to one of the other alias address's. Is anyone else seeing this behavior? I can even force it to happen by logging into Outlook Web Access, going to options, pulling up the phone and deleting the device. The next time the phone tries to contact the Exchange server, it picks up the wrong address and populates it in the email address field under settings.