The first problem is that the subdomain "mail" doesn't exist:
# nslookup mail.troop4051.org
** server can't find mail.troop4051.org: NXDOMAIN
You need to create this subdomain in your DNS Control Panel or Zone file. It looks like you have an issue with authentication... (obviously)... but in order to trouble shoot I'll need the output of the server's mail log. On Debian this should be obtained like so
# tail -n100 /var/log/mail.log
This will give you the last 100 lines from the log. Run this command just after trying to authenticate (i.e. send mail and login to webmail) and then post this log to the forum.