[exim] Slow Delivery - Nothing in log

I am tracing some slow delivery of mail, about 20+ minutes

A message comes in, but Exim appears to make no delivery attempt for 20+
minutes.

The logs show the message is accepted, then 20 minutes later it is
delivered. Nothing in the logs during the 20 minutes for that message, no
delivery attempts.

When I look through the logs, the server is delivering other messages to
said domain/server.

What would cause Exim to just hold the message in the queue like that?

It happens for a small % of mail, and the delay is not always 20 minutes.

Anything I can check out in logs or configuration to determine why it is
holding up the delivery of the messages?