Resolve Microsoft Outlook not finding your new listserv domain name.
If you add a custom domain name for your Mailing List, Microsoft Exchange or Microsoft 365 may not find your new domain name.
Example custom domain: listserv.example.com
An email sent to your Mailing List fails or is rejected.
When the email reaches MS Exchange / Outlook, the server does not look into the external DNS to find where to send the email. Instead, it uses it's internal settings and decides there is "no such address".
How to Test
Try sending email from Gmail, Yahoo or any other non-Outlook email account.
If email sent from other accounts works fine, the problem is internal to MS Exchange / Outlook.
How to Fix
First, check "Accepted Domains" and remove if found.
See example below.
Next, add a Contact pointing to the new listserv address.
Custom domain / Mailing List address example: firstname.lastname@example.org.
By adding a new Contact in this way, MS Exchange / Outlook is forced to look up the address in DNS and resolve the address.