Re: [Exim] failed to find hostname from IP problem

<snip>
>
> 550 relaying to webmaster@somehost prohibited by administrator (failed to
> find host name from IP address)
>
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I am wondering, is this message generated by your own Exim, or is it an error
returned from the MTA at "webmaster@somehost" ?

If the former, please refer to other hints already supplied.

If the latter, then the problem may be with your internet provider (ISP) not
providing proper reverse DNS for your IP address, AND the target MTA is set up to
reject on failed reverse DNS lookups. If sending email works to some places, and
not others, this might be your problem.

This is very common, actually. Every time I've had to move my server to another
provider (as, for example, when ISP #1 is bought out by ISP #2), I've had to spend
excessive amounts of time instructing the new ISP on "why it is important to
provide reverse DNS, and would they please do it already."

Some DSL & Cable providers, in particular, have automated schemes, providing
"dummy" reverse lookups for all IP's they assign. A reverse lookup test, that
checks only for the existence of a valid reverse DNS entry, works fine. But if
anyone should go the next step, and reject if reverse DNS hostname(s) do(es) not
match the original hostname, then all heck breaks loose. In fact, it would
destroy any chance of working with servers that provides virtual hosting for large
numbers of domains, plus any single host connected to the web via such an ISP (one
with dummy reverse DNS entries). I suspect the majority of domains fall in one of
these categories. Thus, I recommend against taking that last step, should anyone
be thinking about it. (Except for internal-only networks, of course).

OK, thanks for letting me spout for a bit, even though this might not solve your
problem.

Jim Roberts
Punster Productions, Inc.