Mautic has a feature where not only is a bounced email added to the bounced-emails segment, it also has a Bounced Flag, that has to be reset for you to email a user again. You can remove it one by one for each user, however if you have hundreds of thousands of users, you will need a more efficient way.
Here is that way
Log into your Mautic Server
Log into your Webmin Server
Click MariaDB Database Server
Select the Database
Click the Execute SQL button at the bottom
DELETE FROM lead_donotcontact WHERE reason = 2
The flag will be reset
Back in Mautic..
Go into Contacts
In the Filter box enter in:
Select All contacts and remove them from the segment (DO NOT DELETE THEM)
and you can now email them again….