New-TransportRule for appending disclaimers - powershell

Question

This is pretty straightforward. I tried creating a rule in the EAC for sent messages, but it's not working as intended. Basically I need to create a rule that checks if any recipients (To, CC or BCC) are from outside my org. If there are any, append a disclaimer to EVERYONE in the recipients (including inside the org).
Doing this via the EAC doesn't work in the sense that when I specify the rule "If the message... is sent to 'Outside the organization' " it finds ONLY recipients outside the org and appends the disclaimer to them. However, I also need to append it for users inside the org if this condition verifies. Unfortunately doing it like this only recipients from outside the org are receiving the appended disclaimer, but not company workers.
I've been working with PS quite a lot lately and New-TransportRule seems to be the way to go to do this, but reading through the documentation hasn't helped me a lot on how to structure said query to do exactly what I want or even how to only apply it to one person for testing purposes.
Any of you guys worked with this cmdlet before and could give me a quick hand?
Thanks!

Answer 1

It's been some time since I've managed Exchange, but judging from the documentation, I'd say you could probably take advantage of the AnyOfRecipientAddressMatchesPatterns predicate (emphasis added):
-AnyOfRecipientAddressMatchesPatterns
[...]
The AnyOfRecipientAddressMatchesPatterns parameter specifies a
condition that looks for text patterns in recipient email addresses by
using regular expressions. You can specify multiple text patterns by
using the following syntax: "Regular expression1","Regular
expression2",..."Regular expressionN".
A match for this condition applies the rule action to all recipients
of the message. For example, if the action is to reject the message,
the message is rejected for all recipients of the message, not just
for the specified recipients.
So let's start by constructing an appropriate pattern:
# Define internal recipient domains
$internalDomains = 'ress.tld','internal.ress.tld','ress-alias.tld'
# Escape as literal regex pattern
$internalDomains = $internalDomains |ForEach-Object { [regex]::Escape($_) }
# Embed in negative look-behind pattern
$nonInternalDomainPattern = "(?<!#(?:$($internalDomains -join '|')))"
The resulting regex pattern will match any email not having any of the three domains listed:
PS ~> $nonInternalDomainPattern
(?<!#(?:ress\.tld|internal\.ress\.tld|ress-alias\.tld))$
PS ~> 'ress#ress.tld' -match $nonInternalDomainPattern # doesn't match on internal recipients
False
PS ~> 'iisresetme#example.org' -match $nonInternalDomainPattern # but it matches any other address
True
Now you just need to include it in a transport rule:
New-TransportRule "Disclaimer on all external communications" -AnyOfRecipientAddressMatchesPatterns $nonInternalDomainPattern -ApplyHtmlDisclaimerText '<your>html goes here</your>'