Telnet isn't enabled by default in Vista so if you need to get it working to test your email connections you do as follows:
# Start >> Control Panel >> Programs and Features
# Select Turn Windows features on or off.
# Select the Telnet Client option and press OK.
Now you can use telnet as follows:
# Start >> Run >> Type in cmd and press enter
# Type in telnet mailservername XXX and press enter
where mailservername is the mailserver's name\address and XXX is the port - 25 for sending emails, and 110 for POP3 receiving emails (143 for IMAP receiving emails)