[Exim] Curiosity: what MUA makes ids like...

This was rather eye catching as I watched a tail of exim_mainlog:

2003-09-04 11:28:01 19uyps-0007se-00 <= arealuser@???
H=valid-pool-name (computerno2) [valid-pool-address] P=esmtp S=3423
id=!~!UENERkVCMDkAAQACAAAAAAAAAAAAAAAAABgAAAAAAAAAfmPrcIv+RkuLc2Ke+I7xv8KAAA
AQAAAA3iZdJEwVpUKdrsVa3l029wEAAAAA@???

Can someone tell me what piece of MUA-ware generates such ids? [Sending
host is clearly one of our dialup users' machines--one might guess it's
number 2 ;-).]

This is just curiosity.

--John