Q) I have sendmail installed and operating but it won't let me log in with my email program and check my email|
A) Sendmail is an email "sending/recieving" server, not a pop3 or imap server. You will need to install imap or pop3 (depending on which mail system your using) on your machine.
Q) Sendmail gives me the error "550 - relay denied" when I try to send outgoing mail
A) Make sure the ips of each machine that will be allowed to send outgoing mail is listed in relay-domains file usually found /etc/mail