If you choose to send mail through another domain's SMTP servers:
6. 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).
7. Click Add account >>
8. Open your other account and either click the link in the message Gmail sent or enter the confirmation code in the Accounts section of your Gmail settings.
Please note that your other email provider must provide authenticated SMTP support in order 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, but we've reached out to Yahoo! to try to get it working.