Hi @Homayoun Shokri ,
For example, some groups should not be able to send emails to other groups or
some groups can only send emails to a single user or group.
Agree with both Andy and BOURBITA that you can create mail flow rules based on your needs.
Let's say your requirement is that the members in group SG1 can only send mails to User2, you can create a mail flow rule as follows:
More about Mail flow rules, hopefully you can find the document below helpful:
Mail flow rules in Exchange Server
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