Hi @Brett Toner
I understand that you need to activate an old user email address which is not a domain you use very often anymore, but it seems like it is still active.
You can check for problems with your domain by checking its status. Go to Setup > Domains and view the notifications in the Status column. If you see an issue, select the three dots (more actions), and then choose Check health. The pane that opens will describe any issues occurring with your domain.
Try to track down issues with your domain's DNS setup. The domains troubleshooter in Microsoft 365 will show you any records that you need to fix, and exactly what the records need to be set to.
- Go to Setup > Domains.
- View the notifications in the Status column.
- If you see an issue, select the three dots (more actions), and then select Check health.
- The pane that opens will describe any issues occurring with your domain.
Follow the document for more information: https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-with-domains/find-and-fix-issues?view=o365-worldwide&source=recommendations#services-not-working-with-your-domain
Hope this helps. Do let us know if you have any further queries.
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