Hi wildirishtime,
From the description “The primary domain seems to be the FIRST domain I setup, but when I signed up I was only testing”, do you mean that you set up two Office 365 accounts or verify several custom domains on Office 365 portal?
The bounced notices can result from the different level of the general server, such as FOPE, Organization, Recipient and so on. If you are using several custom domains on Office 365 portal, you can try to follow the following steps.
1. Log into the Admin Page with Admin account.
2. Click the company name on the upper right quarter of the page.
3. Click the Edit on the dialog box.
4. Beside Default domain, click the drop-down list and select the domain which you want.
Thanks,
Ida Qiu