Until now, when you sent emails from Gmail using non-Gmail addresses, Google included your original Gmail username in the "Sender" field, since these messages’re sent by Gmail's servers. Most email programs just display "From" address and not the "Sender" field, but some (including Outlook) show these messages as coming "From username@gmail.com On Behalf Of customaddress@mydomain.com" which annoyed people. To avoid this, now instead of using Gmail's servers to send the message, Google'll use the servers where your other email address lives to avoid including "Sender" info and "on behalf of" in the header. To switch to this new method, go to the Accounts page under Settings, and click "edit info" from the "Send mail as" section. Then choose the option to "Use your other email provider's SMTP servers." If you use Google Apps Premier or Education edition and would like to send mail as another address within your domain or within an aliased domain, you can do so.