Cannot Access Email On One Network

I had the email host setup a test account for me to use. I have tried logging in to the email account on four different networks. On three of the four networks, I can successfully log in to webmail. On the fourth network, I cannot. Why would one network block access to this certain pop host? I am running an Apple Airport Extreme router, and I have not set it to close any ports--at least not knowingly . Need getting pop mail to work on a network?