I guess I'll dip my toes in here and admit that I can't figure this out.

Synopsis:  I've setup Postfix to be a mail out only smtp server. I just want
it for our Joomla based web pages and our helpdesk to be able to mail to
users from the local server. The problem is this: I can mail to any domain
(gmail, hotmail, yahoo, etc) without problem, but I can't receive mail *
directly* through our A-plus based mail (the worst!). What happens is that
A-plus' server rejects the user as unkown even though I know it is correct.
What really gets my goat is that after that message is returned to my server
it is then delivered to the A-plus server and thusly shows up in my webmail
and email client. Below are my configs.

Postfix:

#soft_bounce = no
queue_directory = /var/spool/postfix
command_directory = /usr/sbin
daemon_directory = /usr/lib64/postfix
mail_owner = postfix
#default_privs = nobody
myhostname = abulafia.ipr.edu
#myhostname = virtual.domain.tld
mydomain = ipr.edu
myorigin = $myhostname
#myorigin = $mydomain
inet_interfaces = all
#proxy_interfaces =
#proxy_interfaces = 1.2.3.4
mydestination = $myhostname, localhost.$mydomain, localhost
#local_recipient_maps = unix:passwd.byname $alias_maps
#local_recipient_maps = proxy:unix:passwd.byname $alias_maps
#local_recipient_maps =
unknown_local_recipient_reject_code = 450
mynetworks_style = host
mynetworks = 168.100.189.0/28, 127.0.0.0/8
#relay_domains = $mydestination
#relayhost = mail.ipr.edu
#relayhost = smtp.comcast.net
#relay_recipient_maps = hash:/etc/postfix/relay_recipients
#in_flow_delay = 1s
#alias_maps = dbm:/etc/aliases
#alias_maps = hash:/etc/aliases
#alias_maps = hash:/etc/aliases, nis:mail.aliases
#alias_maps = netinfo:/aliases
#alias_database = dbm:/etc/aliases
#alias_database = dbm:/etc/mail/aliases
#alias_database = hash:/etc/aliases
#alias_database = hash:/etc/aliases, hash:/opt/majordomo/aliases
#home_mailbox = Mailbox
#home_mailbox = Maildir/
#mail_spool_directory = /var/mail
#mail_spool_directory = /var/spool/mail
#mailbox_command = /some/where/procmail
#mailbox_command = /some/where/procmail -a "$EXTENSION"
#mailbox_transport = lmtp:unix:/file/name
#mailbox_transport = cyrus
#fallback_transport = lmtp:unix:/file/name
#fallback_transport = cyrus
#fallback_transport =
#luser_relay = $user@other.host
#luser_relay = $local@other.host
#luser_relay = admin+$local
#header_checks = regexp:/etc/postfix/header_checks
#fast_flush_domains = $relay_domains
#smtpd_banner = $myhostname ESMTP $mail_name
#smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)
#local_destination_concurrency_limit = 2
#default_destination_concurrency_limit = 20
debug_peer_level = 2
#debug_peer_list = 127.0.0.1
#debug_peer_list = some.domain
debugger_command =
     PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin
     xxgdb $daemon_directory/$process_name $process_id & sleep 5

sendmail_path = /usr/sbin/sendmail
newaliases_path = /usr/bin/newaliases
mailq_path = /usr/bin/mailq
setgid_group = postdrop
html_directory = /usr/share/doc/postfix-2.4.5/html
manpage_directory = /usr/share/man
sample_directory = /etc/postfix
readme_directory = /usr/share/doc/postfix-2.4.5/readme
home_mailbox = .maildir/