RESOLVED: RE: wrong From: and Return Path: address

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RESOLVED: RE: wrong From: and Return Path: address

Michael Fox
> > Michael Fox skrev den 2017-09-21 19:52:
> > > I have a problem that seems to have started when I upgraded from
> Ubuntu
> > > 14.04/Postfix 2.11.0 to Ubuntu 16.04/Postfix 3.1.0.  It involves the
> > > From:
> > > and Return Path: addresses seen by recipients of mail sent from a
> > > virtual
> > > domain on that machine.

This problem turned out to be a DNS issue.  The Postfix 3.1.0 machine's
virtual domain in the From: address was entered in DNS with a CNAME record
pointing to the gateway machine (because they are, indeed, the same
address).  When it was changed to an A record, the problem went away.
Evidently, the user that was certain that the problem only started after the
upgrade was, uhm, mistaken.  The problem also didn't occur on a separate
Postfix 2.11 machine because it's DNS entry was correct.  

BTW, the mail provider found that the default sendmail config and their own
customized config both rewrote the From: header when the From: address was
for a domain that had a CNAME record.  They said this is a config option in
sendmail and they prefer to operate this way.  Interesting that my other
Postfix machines don't do this, nor do many other large email providers.

Michael


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Re: RESOLVED: RE: wrong From: and Return Path: address

Viktor Dukhovni

> On Sep 26, 2017, at 11:23 PM, Michael Fox <[hidden email]> wrote:
>
> BTW, the mail provider found that the default sendmail config and their own
> customized config both rewrote the From: header when the From: address was
> for a domain that had a CNAME record.  They said this is a config option in
> sendmail and they prefer to operate this way.  Interesting that my other
> Postfix machines don't do this, nor do many other large email providers.

This is an obsolete option in Sendmail that violates decades old SMTP
standards, CNAME canonicalization of recipient domains went away in
RFC 2821 and there are many email domains that are aliases where the
mailbox address is expected to remain unmangled by CNAME expansion of
the domain part.  The operators of the sending system are stuck in the
distant past.

--
        Viktor.

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RE: RESOLVED: RE: wrong From: and Return Path: address

Michael Fox
> > On Sep 26, 2017, at 11:23 PM, Michael Fox <[hidden email]> wrote:
> >
> > BTW, the mail provider found that the default sendmail config and their
> own
> > customized config both rewrote the From: header when the From: address
> was
> > for a domain that had a CNAME record.  They said this is a config option
> in
> > sendmail and they prefer to operate this way.  Interesting that my other
> > Postfix machines don't do this, nor do many other large email providers.
>
> This is an obsolete option in Sendmail that violates decades old SMTP
> standards, CNAME canonicalization of recipient domains went away in
> RFC 2821 and there are many email domains that are aliases where the
> mailbox address is expected to remain unmangled by CNAME expansion of
> the domain part.  The operators of the sending system are stuck in the
> distant past.

Thanks Viktor.  Yes, I see section 3.6 now explicitly allows "CNAME RRs
whose targets can be resolved, in turn, to MX or A RRs", which was the case
in my situation.

Good to know that Postfix does it better!

Michael


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Re: RESOLVED: RE: wrong From: and Return Path: address

Viktor Dukhovni
On Wed, Sep 27, 2017 at 07:00:39PM -0700, Michael Fox wrote:

> > This is an obsolete option in Sendmail that violates decades old SMTP
> > standards, CNAME canonicalization of recipient domains went away in
> > RFC 2821 and there are many email domains that are aliases where the
> > mailbox address is expected to remain unmangled by CNAME expansion of
> > the domain part.  The operators of the sending system are stuck in the
> > distant past.
>
> Thanks Viktor.  Yes, I see section 3.6 now explicitly allows "CNAME RRs
> whose targets can be resolved, in turn, to MX or A RRs", which was the case
> in my situation.
>
> Good to know that Postfix does it better!

Even Sendmail does it better in default and recommended configurations,
the users in question are deliberately choosing to continue no longer
sound practices.

--
        Viktor.