Have you tried using the Admincp -> Email Integration -> Forum Settings Help to help determine your mailbox settings?
Your issue is that your hostname, username or password is not correct for the settings you have entered for forum ACCD
You can use the @ symbol just fine, however try without that as well.
If it's on the same box as the server you can try the hostname of localhost.
Also, do you have IMAP installed? or just php enabled? If you have IMAP installed, you'll need to use the IMAP settings instead of the pop3 you have selected.
Worst case, if you can't get the forum settings help script to work, gimme a hollar and I'll help determine what your settings should be as I have time