[exim] Why would this happen?

So my ISP uses exim and I use .forward to call a perlscript to further
filter my mail.

What I observe is that whenever my ISP has an issue with their mail
server a queue will build and all the senders of the messages that
couldn't be delivered to me will eventually retry delivery. What
surprises me is that although I would normally expect to see messages
eventually arrive on their own when their queue is flushed.
Occasionally it requires me to send myself a test message to get
things moving. That suggests to me some misconfiguration of exim such
that the queue or delivery of messages blocks at some point and
unblocks when I send myself a test message. So whenever I send myself
a test message I notice a lot more mail arrive out of sequence than
would have had I not had sent myself the message.