In the past, when you had add the email accounts that are not using “gmail.com” domain in Gmail, you can send the email on behalf of that. The recipient received the email from “From username@gmail.com On Behalf Of customaddress@mydomain.com”. This looks weird and not friendly.
If your custom domain email support SMTP, you can now change the settings in Gmail. Click here to add the SMTP servers and it will work fine. When you send email now, you will no longer see the “On Behalf Of”. Gmail now works like a standard email client.
[via Official Gmail Blog]
It isn’t quite fixed yet. If you send from another Gmail address, there are still different Sender: and From: fields, and there’s no option in account settings for “Send through gmail.com SMTP servers”.