[Exim] Exim hicup?

Exim 3.16 on Solaris
Our usually very quick mail hubs seemed to hit a hicup today, one message wasn't delivered for over half an hour.
The logs report that it was continuously busy at this time, is there any way to increase the probablility of a delivery attempt being made on an older email that hasn't been tested?
Anyone else seen anything like this?

Zed