I can't seem to find it so perhaps there isn't a way but does anyone know how (or if) to add multiple header field matches to an Exchange Online rule?
I'm talking about when you create a rule and select "A message header matches these text patterns". Is there any way to add the predicate multiple times? What if I wanted to match an email based on two or three different header fields. Once you've selected that option you cannot add it again and it only seems to support one value for the header field name.
Yes, one rule condition can be used only once in a specify rule, this is by design. You can try create multiple rules to achieve your needs.