Am 11.04.2018 um 16:07 schrieb Ivan Kovachev via dmarc-discuss:
> 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.

Hello Ivan,

I'm unrelated to the companies but had a similar issue.
A customer use a domain hosted at Mimecast and forward to us. DMARC validation 
failed for a portion of messages sent from p=reject domains.
I had to disable "reject on DMARC fail" for the servers sending from Mimecast 
to us. That's fragile as Mimecast may change them at any time.

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