How to restrict receipient emails in Exchange Server. (I want to exclude some recipient (not in the domain) to receive emails by blocking them at the server level

Set-RecipientFilterConfig –BlockedRecipients "user@contoso.com"

If you're sure you want to do this at the server level, you can create a Transport Rule:

Conditions:  sent to people
(After adding this condition, click the link at the bottom of the window to select specific people (recipient emails) that this rule will apply to)
   AND
Conditions:  when the From address contains specific words  (or use 'when the From address matches text patterns' if that's more relevant)
(After adding this condition, click the link at the bottom of the window to add the specific domain you are looking to block)


Action: Delete the message without notifying anyone
    OR
Action: Send rejection message to sender

The Transport Rule can be highly targeted in that it can be programmed to only affect certain recipients.

When checking for specific words, you can use phrases and also regular expressions.
Here is a link with some examples:
https://docs.microsoft.com/en-us/office365/SecurityCompliance/use-transport-rules-to-configure-bulk-email-filtering

Tks for your answer.

"Set-RecipientFilterConfig –BlockedRecipients "user@contoso.com" "

-BlockedRecipients
The BlockedRecipients parameter specifies one or more SMTP addresses. To enter multiple SMTP addresses, separate the addresses by using a comma. The maximum number of individual SMTP addresses that you can input is 800.

We could have more... :(