In the Accounts section, go to the properties of your email account. Then in the Servers area, (a tab which is at the top of this window)
check that the details are correct. That there is something like this in the POP3 section:
And the the SMTP section, like this:
Also, sometimes the POP3 info goes to the Username section and has a slash after the username. Take the slash out and the POP3 info and just leave the username.
Click on OK. Then try Send/Receive again.