mailing list / pop3 issues

We have a mailing list client that we use to maintain and process
(send / receive) are mailing list of over 6000 folks. The list client
uses a mailbox on exchange. We use our internal nic card IP as the
pop3 and smtp server address. This has worked consistently in our
system for many years. The last few days I have been trying to send
out our mailing list to no avail. The program log says that it has
connected up to the exchange server ok and has sent all the mail
correctly, but the mail is never received nor does it register in the
SMTP queues (as I disable it to see the mail accumulate). The program
will successfully send and receive single messages, but not the full
mailing list. Anybody got any ideas? You assistance is appreciated.

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