Having Trouble with POP mail client

I experienced a POP error with my mail client. I am opening my Microsoft email and this message pops up. The cause is not clear to me so I can’t figure out how to solve it. My mail Post office Protocol is having problems delivering or downloading emails. I would like to ask for some suggestions on how to resolve POP errors. I could not just let this error stay here for long.
Thanks very much for the help.
