Unrelated to Debian. Telnet

[Wayne]

Hi All,
I search the web for an answer, but came up short.
Occasionally I get some junk mail that  will hang
my system and won't let me get my mail. So I
telnet into my isp an look for the culprit. The
command I use is "retr #" <---"#" is the message number.
What command do I use to delete the said message.
When I do "?" or type "help" I get command
unknown. I called my isp to find out what command
they use, they did'nt know. Also, can anyone
recommend a good book on the subject?
Thanks.
Wayne