Gmail – Change personal Gmail account to use own domain name

I'm certain you use to be able to specify a custom outbound server, but it appears you can only send through the server you are logged into.

Using multiple account signin might be an option

Have you tried following the account deletion procedure?

To delete your Google Account, follow these steps:
1.Sign in on the Google Accounts homepage. (If you forgot your password, you can reset it).

2.Click Edit next to the "My products" list. If you don't see the Edit link, your account was likely created through an organization or company. To delete your account, talk to your domain administrator.

3.On the following page, click Close account and delete all services and info associated with it to delete your account.

4.Confirm your account deletion. To do so, you'll need to select these two options: "Yes, I want to delete my account," and "Yes, I acknowledge that I am still responsible for any charges incurred due to any pending financial transactions." (You can safely select the latter option if you haven't used any of Google's paid products, such as AdWords and Google Checkout, or if you have no pending financial transactions related to these products.)