[exim] HELO verification

Hi, all,

I'm looking for another way of knocking down spam. I've got the usual HELO/EHLO
verification tests - rejecting a bare IP or someone claiming to be my domain.

However, more of the spam getting through has a HELO/EHLO name containing a
random string of characters.

What I'd like to do is test the name for a valid domain name format - not
necessarily validate the domain itself, just the format.

For example, a $sender_helo_name of "example.com" or "mail.example.com" could
get through fine.

So I'd like to check for basically anything which has one or more periods
non-consecutive periods. Also, following the last period should be two to four
(or is it 5? I don't remember offhand all the possibilities) characters.

So the above two would work, but something like "asderg" or "14509284" would
not. Neither would "sdfkeoi.llsowwg".

Unfortunately, I'm not good enough at exim configuration to figure out how to do
this.


On a similar topic - I'm considering rejecting mail from certain country TLD's
which would never send mail. Not to mention any names, but I get a fair amount
from .cn, .pl, .br and a few others. How could I reject mail based on a list of
TLD's?

TIA for any help you can provide. Wish I understood this better!