Re: [Exim] Newbie SMTP/ISP-Problem ...

James P. Roberts wrote:
> Even if I could get my IP addresses properly delegated (ie. able to
> control the reverse DNS entries), you still wouldn't necessarily get
> a match, because you could get MULTIPLE answers to a reverse
> DNS query, one for each domain hosted. If there are a LOT of
> answers, DNS will truncate the results. BTW it is perfectly
> legitimate to get multiple answers to a reverse DNS query.
> Happens all the time. As it should. Face it, we can't
> afford to have one unique IP per domain. There aren't enough to
> go around anymore. Virtual hosting is a virtual necessity!
>
> If I understand your comments correctly, (and I very well may
> not; it's >5:30 am and I've not been to bed yet!), you may be
> blocking dramatically more email than mere spam. I would
> hazard a guess that more than half of all email users do not have
> matching forward and reverse DNS entries. (Not even counting
> spammers). Does anyone have any hard data on this?
>

If this discussion is about checking reverse IPs in the Exim fashion,
this is not quite what it does. Rather, what it does is:

- do a reverse lookup on the IP address to get (possibly a list) of
names for that IP
- do forward lookups on the names returned by step 1, and remember all
of them that match the incoming IP address
- for the ones that matched, see if any of them match the HELO string.
if none of them do, it reports the HELO string and the looked-up name in
the log

At no time does this stuff compare to domain names present in the
message headers, or MAIL FROM:.

So, it doesn't matter that your machine has reverse DNS that resolves to
some weird name. As long as that weird name resolves the same IP, you're
all set. That's how both the servers I manage operate, since we have
single static IPs from our providers (one cable, one wireless) and have
no say in reverse DNS at all.