What happens if say the primary server name is set to mail.blah.com,
and there is an alias for blah2.com.
In the alias file for blah2.com there is:
alias1: alias2
alias2: account1
If somethings comes in for alias1@???, it tries to match alias2
as an account or in the alias file mail.blah.com. When it does not
find it in either, then it rejects the email.
I think that perhaps, when checking alias file, that when exim
encounters an alias, it looks through the same file to find another
match, before looking else where.