Because other people might be using your PC - but there's a way around it.
I'm assuming that you are not being blocked because you're not the
computer's administrator and that it isn't an Exchange Server environment
where you have to live by the rules they've established.
Control Panel > Mail > Servers. Input your Incoming Mail server account
name and password and click remember password. If your outgoing mail server
requires authentication as well, click the box at the bottom and then click
Settings. Again, click log on using and input the appropriate account name
and password and click remember password.
Click OK until you get back to the control panel. and that should do it.
