[Exim] Frozen deferred messages

Why does Exim sometimes send messages with the subject "Message
frozen" to the administrator, concerning deferred messages that
are nowhere near as old as the retry rules (as I understand them)
seem to allow, and also have a valid return address?

# exim -brt user@???
Retry rule: * * F,2h,5m; G,16h,1h,1.5; F,1w,6h;

Sample "message frozen" message (signal 14 seems to be SIGALRM):

Message 1BD98w-00043H-Ko has been frozen.
The sender is <example@???>.

The following address(es) have yet to be delivered:
user@???: smtp transport process returned non-zero status 0x000e: terminated by signal 14

-aw