Re: Lenny/exim4: how to set helo for outgoing SMTP
Joe wrote:
[...]
> As it happens, the PTR-A pair for my IP address correspond to my
> subdomain at my ISP, and neither bear any relationship to any of the
> domains I actually use for email. I'm not aware of any email domain
> which refuses my email, though the only 'difficult' one I send to
> regularly is AOL.
As the other poster already wrote, of course neither the A-record nor
the PTR-record nor the HELO value must be in any way related to the MAIL
FROM: value, but this is not the problem.
> It is alleged that there are some mail admins who require HELO and PTR
> to match, but to me that seems silly.
Well I am one of those admins; to me, this is simply a measure of SPAM
control. If you want to send email on the internet, you need:
1. a server with a public IP address
2. an A record which resolves to that IP address
3. a PTR record for the IP which resolves to the hostname of the server
Furthermore, RFC 821 states that the argument field of the HELO command
contains the host name of the sender-SMTP. So, in a proper setup, A-RR,
PTR-RR and HELO value are identical.
> While the specification for PTR
> records allows for multiple PTRs for one IP address, I think in practice
> there's little software which can deal with this. So anyone sending mail
> for multiple domains from one IP address will have unmatched PTR and
> HELO strings most of the time.
IMHO: no, HELO always contains the hostname of the sender-SMTP,
regardless of the MAIL FROM: address.
> The difficult hurdle for the spammer in control of a home computer to
> jump is the PTR-A pair setup, as one must be set by the domain host and
> the other by the ISP. Neither are under user control.
... which is exactly why you should check that all three values match,
shouldn't you?
-- Kilian
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