Receiving Google Groups mail at my personal Google Account
If you're suddenly receiving mail from Google Groups to the primary email address listed on your personal Google Account, this is because you're unable to use your @mydomain.com address as an alternate email addresses.
If your organization signs up your @mydomain.com domain for Google Apps and you previously had the @mydomain.com address listed as an alternate address on your personal Google Account, the address will be removed as an alternate email address on any Google Accounts you've set up. If you've set any groups to send email to your alternate email address, emails from Google Groups will now be delivered to the primary email address on your Google Account.
You can check your email delivery settings for your groups by visiting the Google Group and clicking 'Edit my membership' on the right side of the page.