POP3 Error "-ERR Server Unavailable. 21"

[POP3] Message listing failed due to the following reason:
“-ERR Server Unavailable. 21”

The above error occurs every time I open eM Client. I am running Windows 11 and have recently set up Windows defender as my AV, could this be causing the problem?
Regards
Buzzbird

This problem has now resolved itself. Thank you for your interest’