[exim] SMTP authentyication

We are using SMTP authentication with "plaintext" and "pam". It all worked
fine when the userids and passwords were the normal short unix ones.
However with Active Directories we are moving to longer passwords and
these seem to fail - at least a 10 character one is reported to fail.

I can see EXPAND_MAXN referred to in auths code with

config.h.defaults:#define EXPAND_MAXN 20

and wonder if this is the cause of the problem and whether I should
increase it via Local/Makefile.

John Linn