Open Specific External Domain for sending to blocked DLs

[DrFunk]

I'm looking for a solution to open an External domain to have the ability to send to Distribution Lists, which are blocked for External domains. I know we can do this, via a Transport Rule that targets the specific DLs; however, we have over 12,000 DLs and cannot add them all into a single Transport Rule.  Is there a way, in Exchange Online, to tag a specific external domain (without adding it as an Accepted domain) as "Internal", so it can send to these blocked DLs? 

Thank you for your time! 

[#2]

Hi @DrFunk,

1) could you please explain in details how can you do that:

"via a Transport Rule that targets the specific DLs" ?

2) > Is there a way, in Exchange Online, to tag a specific external domain (without adding it as an Accepted >domain) as "Internal", so it can send to these blocked DLs? 

No way to tag as far as I know.

The only way I see is using of combination of a transport rule and a custom Power Automate flow.

[#3]

@Victor Ivanidze The Transport Rule method we're using is basically this: 

Apply this rule if - The To or CC box contains... <all of the targeted DLs> 

Do the following... Reject the message with the explanation... <This is an Internal Distribution List>

Except if... The sender's domain is ... <specified domain - primary domain of o365 Tenant>

[#4]

Hi @DrFunk,

ok, understood.

Q1. do you have the DLs that should behave differently? In other words, can I assume that each your DL should reject external messages with exceptions?

Q2. if the list of external domains that can deliver to your DLs is the same for all DLs or different DLs have different lists of exceptions?

[#5]

Hi, @Victor Ivanidze 

For the DLs, in question, they are blocked from receiving email from External domains.  The External domain that we want to allow is from a single tenant.  

[#6]

Hi again @DrFunk,

well I see 2 options then:

 1) (cheap & ugly)

   a. For each of 12000 DL you have to set a Custom attribute "attr1".

   b. Create a dynamic distribution list named allDLs whose members are DLs with custom attribute "attr1"

   c. Use allDLs in your transport rule

2) (pretty elegant but pricey)

   create a customized Power Automate flow and run it in cooperation with a single transport rule