The following is is a posting from the Official Gmail News Blog:
Posted by Emmanuel Pellereau, Software Engineer
Quite a few of you use Gmail’s custom “From:” to send messages with one of your other email addresses listed in place of your Gmail address. Since these messages are sent by Gmail’s servers but “from” a non-Gmail address, we have to include your original Gmail username in the “Sender” field of the message header to comply with mail delivery protocols and help prevent your mail from being marked as spam. Most email programs just display the “From” address and not the “Sender” field, but some (including versions of Microsoft Outlook) show these messages as coming “From firstname.lastname@example.org On Behalf Of email@example.com” which really annoyed people.
We heard your request for another option that wouldn’t show the “on behalf of” text loud and clear, and now there’s a new option that does just that. Instead of using Gmail’s servers to send the message, we’ll use the servers where your other email address lives. Since Gmail isn’t the originating domain, we don’t have to include “Sender” info in the header. No more “on behalf of.”
Here’s the difference. All custom “From:” addresses used to work like this:
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.”
We recognize that your other address might not have a server that you can use to send outbound messages — for example, if you use a forwarding alias rather than an actual mailbox, or if your other email provider doesn’t support authenticated SMTP, or restricts access to specific IP ranges. For this reason, we’ve kept the original method as well. Check out our Help Center for further details on these two “send mail as” configuration options.
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, no sweat. We do all the work behind the scenes so your original username won’t be listed in the “Sender” header, and your recipients won’t see “on behalf of.”
Read more here:
Send mail from another address without "on behalf of"