Problem with Mail Server

Hi,

I have a problem with a server that delivers mailing list emails for the company I work for. It normally works fine with the mailing list software delivering messages into the sendmail queue and being sent out as and when sendmail says so.

To there are a large number of items in the mail queue in the following format:

5CC184B136 6629 ?? reply@reply-address.com

What does this mean?
How do i go about resolving?

Any help would be great.

Thanks,
David