Exim pending queue growing steadily

Curiously, within the past hour, the Exim 'pending queue' (as displayed within MailScanner ConfigServer Mail Queues application) has been steadily growing. A small amount of mail IS being delivered, but the lion's share must be adding to that growing pending queue.

We've reinstalled MailScanner and upgraded ClamAV, without any apparent effect on the problem.

I suppose I could remove MailScanner completely, to see if that has an impact on the queue issue, but I hate to allow a deluge of spam to the 100 or so clients on this particular box.

I wonder if anyone has any suggestions?

Thanks so much in advance -- Bill