[Exim] Why does Exim need a numeric EXIM_UID ?

Hi Philip,

I'm having trouble finding the reason why Exim needs a numeric EXIM_UID.
For certain scenarios, it definitely is required, as I've seen for
myself.

However, I don't quite understand why it can't _always_ look up the UID
from the non-numeric string value given in exim_user. Could you lay
this to rest for me?

Ciao,
Sheldon.