Currently Microsoft Flow cannot handle digitally signed email (eg. to extract attachments like PDF - signed too)
The trigger "when a new email arrives in a Shared mailbox" won't fire (I suppose "when a new email arrives in mailbox" will not fire in the same case).
The encoding of digitally signed emails is different (MIME, S/MIME) but that's for those who know it.
It is not well documented those emails won't work and it is a bit hard to explain to a user why.
It should be supported so please vote for that "feature" to be added.
Update : I have opened a ticket for that and now it has been reported as a bug.
No more feedback for now.
Any more updates on this topic?
Do you know if there is an option of tracking bug status?
Thanks a lot in advance
The case is still ongoing at Microsoft...
I had several exchange with them about this.
Sorry, there is no public statement about it.