The first thing would be to double check the settings from your ISP, particularly the port settings and if it needs authentication. I'm guessing that from your descriptiuon that you CAN initiate a message and that the only problem is with a reply?
You also might want to check out an alternative mail client like K-9 or MailDroid.
Based on what you're saying it should be working and there is no logical reason why it isn't. I still think you should try with another mail app. I haven't used the stock mail app since December.