DMARC checking within a service provider doesn't make much sense,
however DMARC reporting probably would when/if you implement it:
* Failure reporting seems odd (because it's always legitimate) until
you recall that part of the purpose of failure reporting is to
discover errors by the domain registrant, particularly including
errors in the DNS zone file, which may or may not be under Office
365 control.
* Aggregate reporting likewise seems like something that would make
sense for inter-tenant communication.
Related question: does Office 365 DKIM sign inter-tenant email? (This
would not be terribly important for end delivery at the addressed
tenant, but would be important for messages that were automatically
forwarded elsewhere.)
- Roland
On 23/04/18 12:55, Terry Zink via dmarc-discuss wrote:
>> 3. Would O365 do DMARC checks for internal emails ie.
>> O365 tenant employee to another O365 tenant employee?
>> And would it send DMARC reports in this case?
I didn’t see this answered, so answering it now.
Office 365 doesn’t do DMARC checks for internal emails since they
don’t leave the network perimeter. Since no DMARC check is done, no
DMARC report is sent (Office 365 doesn’t send DMARC reports anyway,
but if it did, it wouldn’t in this case). There are some advanced
reporting capabilities for Advanced Threat Protection customers that
can quasi-approximate DMARC reports, and you could use Transport rules
in the service to approximate a RUF report. But there’s no official
DMARC reporting at this time.
--Terry
*From:*dmarc-discuss <dmarc-discuss-boun...@dmarc.org> *On Behalf Of
*Roland Turner via dmarc-discuss
*Sent:* Thursday, April 12, 2018 12:57 AM
*To:* dmarc-discuss@dmarc.org
*Subject:* [EXTERNAL] Re: [dmarc-discuss] Mimecast and Office 365
On 11/04/18 22:07, Ivan Kovachev via dmarc-discuss wrote:
Hello guys,
I have three questions for you that I am unsure about and hoping
that someone at Microsoft will be able to help:
First two questions are related to Mimecast acting as inbound
security gateway to O365:
1. When Mimecast acts as inbound gateway solution and it receives
an email, it does DMARC checks and lets the email through to O365
environment. Even if an email passes DMARC checks at Mimecast and
the email is let through, then O365 also seems to also be doing
DMARC checks but both SPF and DKIM fail because of the change that
Mimecast does. As a results DMARC fails. My questions is, what is
the best practice here in this scenario? Is there a way to turn
off DMARC checks at O365? Mimecast suggest that it is whitelisted
in O365 but that means that all the spam will be let through as well.
DMARC checking should only occur at the host referred to be the MX
record as SPF is still relevant for at least some email. I believe
Office 365 has a trusted inbound relays option (i.e. Office 365 trusts
the specified hosts to filter their email) although I can't quickly
find it.
Mimecast is apparently unwilling to change their service to stop
damaging incoming messages that don't breach the policies being
enforced (they unconditionally unpack and then repack every message,
rather than only those whose contents they have reason to modify).
2. Would O365 send DMARC reports back to the sender in the above
case? And, if O365 sends DMARC reports back to the sender then
emails will be shown as originating from Mimecast but failing DMARC.
Yes and yes if you've not listed Mimecast as a trusted inbound relay.
(Assuming that the trusted inbound relays setting is not a figment of
my imagination, one would hope that Office 365 would not set feedback
in this case.)
3. Would O365 do DMARC checks for internal emails ie. O365 tenant
employee to another O365 tenant employee? And would it send DMARC
reports in this case?
Yes and hopefully yes.
- Roland
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