Re: [exim] Undefined variable in eval: (string expansion)

Quoting Toralf Lund:
> What do I get if I do something like
>
> ${eval:$some_variable+1}
>
> when "some_variable" is not actually defined?

An undefined variable expands to nothing (except for acl_m/acl_c
variables, if you have the strict_acl_vars option set), so this is the
same as ${eval:+1}, which expands to "1".

Note that there was a change (which was reverted later) recently
regarding empty variables in numeric comparison operators (see the
changelog of 4.66).