Send mail from a different address or alias
Click the gear in the top right .
Click the Accounts And Import tab.
Under Send mail as, click Add another email address.
In the 'Email address' field, enter your name and the alternate email address you own.
Enter the SMTP server (e.g. smtp.domain.com), your username on that domain, and your password for that account. You may also need to adjust your port setting or SSL setting (talk to your other ISP if you need this information).
Click Add account >>
Open your other account and either click the link in the message Gmail sent or enter the confirmation code in the Accounts and Import section of your Gmail settings.
If Gmail sent a verification email and you didn’t receive it, try checking your Spam or Bulk Mail folders for a message from email@example.com to see if the email ended up in there.
Your other email provider has to provide authenticated SMTP support for you to use this option. We'll use TLS by default, or SSL if you enable it. Many email services that provide POP or IMAP support also offer authenticated SMTP support, and you can likely find your SMTP server configuration instructions alongside information about POP or IMAP. Also, this new version of custom 'From:' doesn't work with Yahoo! Mail Plus accounts just yet.
If the address you’re adding is hosted by Google (either a Google Apps account or a @gmail.com address), your original address will still be included in your email header's sender field to help prevent your mail from being marked as spam. Most email clients don't display the sender field, though some versions of Microsoft Outlook may display "From firstname.lastname@example.org on behalf of email@example.com."
Once You're Set Up
To use one of your alternate sender addresses, click the From link when you compose a new message. If you're replying to or forwarding a message, click the field where your recipients are listed, then click From. After clicking From, you'll see a drop-down menu next to your address, where you can select the email address you'd like to send from.
If you ever need to edit the name, configuration or reply-to address, go back to your settings. Click edit next to the address on your Accounts tab. To delete an address, just click delete.
Note for IMAP/POP users: If you access Gmail through a POP or IMAP email client (e.g. Outlook) and would like to send messages with a custom "from" address, you have two options. We recommend that you configure your email client with two outgoing SMTP servers, one for Gmail and one for your other address. Or, you can use Gmail's outbound servers with a different "from" address. If you've already configured the alternate address, your message will be sent from:firstname.lastname@example.org, sender:email@example.com, regardless of which custom from configuration you chose.
The custom 'From:' feature works only if you already own the account linked to the alternate address. To send mail with a different Gmail username, you must first sign up for that address.