I can't get me e-mails today it says someone might be trying to impersonate the server so not to continue.

I can't get my e-mails today. I got a message saying The certificate for pop.centurylink.net is not valid for that server. Someone could be trying to impersonate the server and you should not continue. Why did I get this message? I never saw this before. I like to get my e-mails. I tried reseting my router and turned off my computer and it still says the same thing. What do I do and how will I see your reply if I can't get the e-mails? Please fix this. My e-mail is toosmiley@centurylink.net Thank you