Re: How to manage business and personal email address for custom domain?

Hi Augie Gonzalez,
Based on your description, Maybe you did some wrong configuration during migration. Now I list steps as below,
1. add your custom domain to office 365.
2. Mark the domain as shared in Exchange Online.
3. Create accounts and  set your custom domain as primary  address, at same time ,set yourdomain.onmicrosoft.com as proxy address.
For example:
primary address: user1@yourdomain.com, proxy address: user1@yourdomain.onmicrosoft.com
Primary address: User2@youdomain.com, proxy address: user2@yourdomain.onmicrosoft.com
4. Change TXT record but don’t change MX record.
5. Configure forwarding from old mail provider to office 365 proxy address(only the accounts that migrated to office 365 configure forwarding, but old mail provider still reserve the mailboxes) .such as:set forwarding from user1@yourdomain.com (on old mail provider)to user1@yourdomain.onmicrosoft.com (on office 365).
Best Regards,
Jacky Ding