[exim] Spool file is locked leading to long retry time

Hi all,

I have a problem with some mail on my system. Whenever a queue runner
comes across a message with a locked spool file, it will not attempt
to deliver the message for another 4 hours or so.

We see the following, and the next thing we see in the logs for this
message would be the delivery some 4 hours later.

<DATE> <ID> Spool file is locked (another process is handling this message)

Has anyone seen similar symptoms and does anyone know of a retry rule
tweak to capture these messages?

Thanks in advance,

D.A.