Create Mail Flow Rule as shown in below steps on Office365 Exchange admin panel
Click Admin then click Exchange to open the Exchange Admin Center.
Click mail flow then click on the Rules tab.
Click the + symbol and click Create a new rule.
Name the rule Block Outbound Emails to External Recipients.
Under Apply this rule if, click the recipient is located....
Click Admin then click Exchange to open the Exchange Admin Center.
Click mail flow then click on the Rules tab.
Click the + symbol and click Create a new rule.
Name the rule Block Outbound Emails to External Recipients.
Under Apply this rule if, click the recipient is located....
Select Outside the organization and click OK
Click More Options to add another condition
Click Add Condition
On the new condition, select the sender is a member of this group
Search and select the group Block Outbound Emails and click OK
Note: Despite the wording stating “member of this group”, you can select a user instead of a group. However, it’s easier to manage and you do not need to wait for the mail flow rule to propagate on 365, which can take up to an hour in my testing.
Click More Options to add another condition
Click Add Condition
On the new condition, select the sender is a member of this group
Search and select the group Block Outbound Emails and click OK
Note: Despite the wording stating “member of this group”, you can select a user instead of a group. However, it’s easier to manage and you do not need to wait for the mail flow rule to propagate on 365, which can take up to an hour in my testing.
Under Do the following, select Block the message then click delete the message without notifying anyone, and click OK
Click Save
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