Mailing List Archive

External mail servers
My email SMTP server is hosted externally.

I have been advised by the openspf.org wizard that the MTA should have
an SPF record.

I have asked the service provider to publish an SPF record but they
reply that:-

QUOTE
It is very rare that a mail receiver client will perform SPF lookups on
anything other than the MX relays of the email sender domain (MAIL
FROM/HELO domain). Most mail receivers will use SPF amongst many other
checks to determine a final action for the mail, however some will
decide to REJECT the mail based solely on the SPF results.

Using your domain as an example, since the SPF check for
mortgage-advice-centre.co.uk (when sent via mail1.ukisp.com) will PASS,
further checks will be un-necessary and just a waste of bandwidth.

According to RFC4408, Section 9, Subsection 9.5,

"The (SPF) authorization check generally precludes the use of arbitrary
MTA relays between sender and receiver of an E-Mail message."

"To perform the authorization test other than at the border,
the host that first transferred the message to the organization must
be determined, which can be difficult to extract from the message
header. Testing other than at the border is not recommended."

You will notice that most organisations will not have an SPF record
published for their domain's MX relays. For example, google.com:

dig mx google.com

[snip google records]

As no mail is generated as being FROM mail4.ukisp.com, the requirement
for an SPF record is neglible. Backscatter in this instance is not a
problem as no valid recipients at mail4.ukisp.com will exist, therefore
be accepted.
UNQUOTE

My question - are they correct?

If they are correct, why is there not more accurate guidance at
openspf.org?

If they are not correct, what is the follow-up that I should send to
repudiate their response and why is this topic not covered in the
openspf.org FAQ's




--
Best regards
Harvey Coward

Slough Mortgage Centre - In your best interest. ®

Mortgages & Loans. Conveyancing. Debt Management.
Life, Critical Illness, Income Protection, Buildings & Contents Insurance.

Abbey House, 18-24 Stoke Road, Slough SL2 5AG
Tel: 01753 516567


E-mail communications are not secure, for this reason Slough Mortgage Centre
Ltd cannot guarantee the security of the e-mail or its contents or that is
remains virus free once sent.

This e-mail message is strictly confidential and intended solely for the person
or organisation to whom it is addressed. It may contain privileged and
confidential information and if you are not the intended recipient, you must
not copy, distribute or take any action in reference to it. If you have
received this e-mail in error, please notify us as soon as possible and delete
the message from your system.

Slough Mortgage Centre Limited. Registered Office "Willsgrove", 10 The Chimes,
Dean Close, High Wycombe, HP12 3HR. Registered in England, number 04460564.
Consumer Credit Licence 570377. Slough Mortgage Centre Limited is an appointed
representative of Modus Mortgage Network which is authorised and regulated by
the Financial Services Authority.


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Re: External mail servers [ In reply to ]
I have two additional questions that this message has triggered:
1) Why aren't Disposition-Notification-To headers removed?
2) Why doesn't jeeves.archives.listbox.com have an SPF record?

Harvey Coward wrote:
> My email SMTP server is hosted externally.
>
> I have been advised by the openspf.org wizard that the MTA should have
> an SPF record.

That serves a twofold purpose: protecting the corresponding email
addresses, and having a valid helo name for bounces (or any message
sent without a MAILFROM.)

> I have asked the service provider to publish an SPF record but they
> reply that:-
>
> QUOTE
> [...]
> As no mail is generated as being FROM mail4.ukisp.com, the requirement
> for an SPF record is neglible. Backscatter in this instance is not a
> problem as no valid recipients at mail4.ukisp.com will exist, therefore
> be accepted.
> UNQUOTE

That addresses the protection purpose.

> My question - are they correct?

Yes, in part. They should also state that no bounces (or any message
without a MAILFROM) are ever sent from that host. I think they have
turned out backscatter for any message that will result to be
undeliverable after having been accepted by mail4.ukisp.com from
external sources, but it may be a good idea to ask for more details.

As a marginal case, a check could be missed from internal sources.
What happens to undeliverable messages that you send? If you write to
unexisting-user@existing-domain, your message may make its way to
mail4 and only there be found to be undeliverable. In that case, a
bounce would be generated with a helo name that misses an SPF record,
thereby producing an output of "none" if the internal receiver runs
SPF checking against its MX, or your client might discard it on a
similar basis.

> If they are correct, why is there not more accurate guidance at
> openspf.org?

That point has never been accurately worked out. The FAQs barely
mention it in http://www.openspf.org/FAQ/The_demon_question



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Re: External mail servers [ In reply to ]
Thanks Alexandro,

(I've removed the MDN request from this message!)


In message <4A48E8CC.60700@tana.it>, Alessandro Vesely <vesely@tana.it>
writes
[]
>
>Yes, in part. They should also state that no bounces (or any message
>without a MAILFROM) are ever sent from that host. I think they have
>turned out backscatter for any message that will result to be
>undeliverable after having been accepted by mail4.ukisp.com from
>external sources, but it may be a good idea to ask for more details.
>
I have informed them of your comments.

As an aside, even if they didn't "need" an SPF record, would there be
any resource or other implications for them if they had an unnecessary
SPF record?

>As a marginal case, a check could be missed from internal sources. What
>happens to undeliverable messages that you send? If you write to
>unexisting-user@existing-domain, your message may make its way to mail4
>and only there be found to be undeliverable. In that case, a bounce
>would be generated with a helo name that misses an SPF record, thereby
>producing an output of "none" if the internal receiver runs SPF
>checking against its MX, or your client might discard it on a similar basis.
>
A "none" output is pretty benign isn't it?

I tried sending a message to kjahwbbceffhjvcbihevh@googlemail.com (I
chanced that there is no such address) and got the bouncer back.


--
Best regards
Harvey Coward

Slough Mortgage Centre - In your best interest. ®

Mortgages & Loans. Conveyancing. Debt Management.
Life, Critical Illness, Income Protection, Buildings & Contents Insurance.

Abbey House, 18-24 Stoke Road, Slough SL2 5AG
Tel: 01753 516567


E-mail communications are not secure, for this reason Slough Mortgage Centre
Ltd cannot guarantee the security of the e-mail or its contents or that is
remains virus free once sent.

This e-mail message is strictly confidential and intended solely for the person
or organisation to whom it is addressed. It may contain privileged and
confidential information and if you are not the intended recipient, you must
not copy, distribute or take any action in reference to it. If you have
received this e-mail in error, please notify us as soon as possible and delete
the message from your system.

Slough Mortgage Centre Limited. Registered Office "Willsgrove", 10 The Chimes,
Dean Close, High Wycombe, HP12 3HR. Registered in England, number 04460564.
Consumer Credit Licence 570377. Slough Mortgage Centre Limited is an appointed
representative of Modus Mortgage Network which is authorised and regulated by
the Financial Services Authority.


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