Mail Flow Rule Question

Hello therekhamilton 
Im a 23 year old Microsoft certified Expert that focuses on Office365, Exchange and Azure. 

I think that Ive managed to create a rule that matches your criterias. 
Please read my instructions below and let me know what you think. Maybe I've missunderstood you. 

1: In ECP (  Exchange Control Panel ) click "Mail Flow" 

2: After that, click on "Create a new rule" 

3: Now we get to the creation. Start by giving the rule a name and then click "More options" To get all the advanced Featues

4: Now, start by adding the first condition as my image below 

"The Subject or body"-->"Subject or body includes any of these words" 

Here you can then specifiy the words you want to look for by typing them and click the "+" sign. 

You can mark a word and click the pen to edit it into a phrase

5: After this, we add the second condition as my image below 

"The Sender"-->"Domain Is" 

In this rule above you type the domain of your client ( Client A ) 

6: Now time for the action... we reject the email like my image below 

"block the message" -->"Delete the message without notifying anyone" 

( You can choose another action that fits your needs better ) 

7: Finally, we add an Exception, this will make that, as long as the recipients of the email are related to the "Client A" domain or your companys domain, The message will be delivered like normal. See images below 

Now, your mail flow rule should look like this 

This rule will look for emails sent my companyA users, that have specific words in them. And block them from sending if the recipients are not within the CompanyA domain, or in your companys domain. 

Kind Regards
Oliwer Sjöberg