What do I do about this warning I am receiving when I refresh my inbox? "The certificate for pop.windstream.net is not valid. Someonecould be trying to impersonate the server and you should not continue"

   Unable to receive new messages on my Thunderbird inbox for this warning

What do I do about this warning I am receiving when I refresh my inbox? "The certificate for pop.windstream.net is not valid. Someone could be trying to impersonate the server and you should not continue" Unable to receive new messages on my Thunderbird inbox for this warning