A brief Exim observation
Life probably would be easier if the people behind Exim's string expansions had just been able to write a Lisp. Unfortunately that might have created some irrational prejudice against Exim, so I can see why they needed to disguise what they were doing.
(It would have been a pretty mutant Lisp. But mutating your Lisp is almost de rigueur.)
Of course, I must include a pointer to Greenspun's Tenth Rule of Programming.