Setting up a forward on a shared mailbox in a hybrid environment for thousands of domains/addresses

I was asked to set up a forward on incoming emails from specific senders on a shared mailbox. We have a hybrid setup and the shared mailbox resides on-premise but the MX record for that domain is already pointing towards Exchange Online Protection.

Incoming emails from 1897 domains and 1581 individual addresses should be forwarded from the shared mailbox to another shared mailbox in the same domain which also resides on-premise , what is the most efficient approach? Do I create a bunch of rules in the on-premise Exchange or do I create them directly in Exchange Online?

Due to the sheer number of domains/users that need to be forwarded, efficiency is key.
Triforce2000Asked:
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MaheshArchitectCommented:
you need to create transport rule in exchange online

You can create mail enabled group of individual addresses and configure it with 1st transport rule to forward to another mailbox

with 2nd rule add all domains as sender and forward it to another shared mailbox, note that EXO allows you to create rule with size restrictions up to 8 KB I believe, so if your 2nd rule size is lesser than 8 kb its fine, else you will get error while creating rule
This limitation is not there with on premise exchange

The best way i can think off is, instead of creating granular transport rule, simply set forwarder on 1st shared mailbox to point to 2nd shared mailbox and let's that be end of it.
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Triforce2000Author Commented:
Hi Mahesh

Thanks for your reply.

I doubt I will be able to fit all those addresses and domains within 8KB so it will have to be multiple rules...

The mailbox forward functionality will not allow me to target specific domains/addresses and creating an inbox rule might have limitations as well.
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MaheshArchitectCommented:
for multiple addresses, you can have add all of them to single DL and let put DL into rule
Problem exists with multiple domains rule. U need to create multiple rules for that so that exchange will evaluate message against every rule one by one
Also you need to alter these rules every time there is an update

I know forwarding will not allow you to target specific domains and addresses, however my point is when you have 1500+ domains / addresses to be evaluated, it is as good as evaluating every sender through which your organization is working unless this email address is distributed on social sites to receive anonymous emails.
Hence its best idea to just set forwarder to another shared mailbox once and forget as I believe this is not high privileged or sensitive account. Shared mailbox is meant to distribute email to multiple users with send rights
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timgreen7077Exchange EngineerCommented:
All these domains and senders, do they all send to this one shared mailbox. Does anyone other than those domains and sender send that that share. So if only those senders and domains send to that share, then just create a forward on that share, and have it to forward all mail to the other shared mailbox also. No need for any transport rules or anything if this share is only used for the Incoming emails from 1897 domains and 1581 individual addresses. Now if the share mailbox also gets emails from others other than those that presents a different issue but if that is not the case, a simple forward on the mailbox properties will work.
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Triforce2000Author Commented:
Hi Mahesh

Forwarding will forward all messages send to that shared mailbox while only a subset of domains and addresses should be forwarded to the other shared mailbox.

What would be the most efficient? Rules on the on-premise Exchange or rules directly in Exchange Online?
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MaheshArchitectCommented:
where mailbox is located?
Since your shared mailbox is residing in O365, create rule in O365
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Triforce2000Author Commented:
Hi Mahesh

The mailbox resides on-premise but the MX records for the domain already points to EOP so transport rules could be setup either way.

@timgreen7077, other domains and senders also send to that shared mailbox.

I created a total of 11 rules on our on-premise Exchange currently (properties are limited to 8192), not the most elegant solution but it gets the job done.
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Triforce2000Author Commented:
I created a total of 11 rules on our on-premise Exchange currently (properties are limited to 8192), not the most elegant solution but it gets the job done.

https://docs.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/use-rules-to-route-email
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MaheshArchitectCommented:
Don't your shared mailbox get emails from internet, those domains and addresses i believe not all internal, they must be external in majority, is not it?
how come your solution works with rules on premise only
Sorry but simply don't understand
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Triforce2000Author Commented:
Hi Mahesh

The shared mailbox still resides on-premise but the MX record is pointing to EOP, as a result the rules can be created either in EO or Exchange on-premise.
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