I'm receiving a fair amount of spam that is coming through Microsoft's
Office 365 service (spammers signing up for the one month free trial of
Office 365).  This provides them a DKIM siguature that is valid and has
a customized subdomain of onmicrosoft.com.    The email itself has a
sender domain that may be entirely different and will likely change.

What I'd like to do is set up an ACL that checks the DKIM signature and
rejects it if the domain is anything ending with onmicrosoft.com.

I tried using a deny rule in my DKIM ACL that looked at the dkim_signers
but I got the implementation wrong because it rejected anything that had
a DKIM validation in it no matter the domain.

deny message = DKIM from unwanted domain
dkim_signers = *.onmicrosoft.com

How would I accomplish what I want if it's even possible?

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