Exch. Srvr. email being sent is not being received by all recipients

My customer is running MS Exch. Srvr 2003 on a Windows 2003 Ent. Ed. srvr.  When they send mail from Outlook 2003, the email shows up as sent in their 'sent items' box.  However some emails are never received by recipient - mine included.  They originally thought it was just my email, but have come to find out that other recipients never received their emails either, although they also show up in the senders 'sent items' box.  However, when I'm logged onto their srvr. thru term. svcs., I am able to send to my external email address( the one they're always sending to) using Outlook Express, which has the Administrator acct. oonfig'd on it.  Prior to a certain date, I was getting emails from them - up until 3/28/06, after that time I received no emails from them although they had sent quite a few.  And I've checked these emails on their wksts., and they showup in their 'sent items' folder.
I've checked the sending of emails while term svc'd onto their server with tech supp. through my email hosting provider(which is external to my office).  Although he sees the emails arrive and be sent from his serverto my email address - as long as they're from the 'Administrator' acct. , when anyone else sends them, they never reach his email server. Which of course means they're never getting out of the customers Exch. Srvr.   I've checked the configurations of the Pop3, and SMTP settings on both Outlook Express and Outlook 2003, and they are the same.  The only difference is the user acct. itself that I'm sending from.  Also, I've run isinteg.exe and eseutil.exe on the information store d'bases successfully.   This problem has me stumped - any help would be greatly appreciated!
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