In order to setup Gmail to send email from these accounts, please follow these instructions:
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 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).
You can find the SMTP information to add into this section here: How To Setup A Third Party Email Application To Access Your Email Accounts
If you're having any trouble with this step, please submit a ticket, and we would be happy to help.
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 firstname.lastname@example.org to see if the email ended up in there.
Also, if you want to make sure you receive an copy of all emails that are sent to the accounts on your site in your Gmail inbox, please make sure you have setup an email forwarder for all of your accounts.