A simple check would be to ping the mail server on the mail port
In a command prompt window type ping pop.servername.com:110 and see if the server is operating
change the address to your server name
Change the number after the colon to the port number the server uses could be 110, 993, 995, 143, 25, 585 or other number check your provider's spec
If you get a not found error then the server is totally down if you get a result then the server is up but mat still be stalled. If you get a cannot resolve error you have a DNS issue