It is the synchronization between Flexmail & Gudsis.
Since Flexmail is associated with Gudsis repository, we can check all the necessary details from the Gudsis data.
To check the GUDSIS data, open the link provided below in a new browser.
file://nohrs89/SOLWEB_Transfer/ToSOLWEB/GUDSIS/
Download the file, where we have details of all users available in Flexmail.