[exim] Performance considerations

I'm trying to desing a mail handling architecture for an ISP (about 20K
users, soon scaling to 40K), the candidates: exim and postfix. The
'claimed drawback' of exim by other members of the team is to be memory
hungry by being monolithic.

I'd like to read some (serious) documentation about what features have
exim to reduce/optimize memory compsuntion due to its condition of being
monolithic.

Regards
mike