RE: How to block sending email another Domains

Hi Suresh,

You need to set up a transport rule to achieve your goal. Here are the detailed steps:

1. Login to Exchange admin center via http://outlook.office365.com/ecp .
2. Click mail flow->rules->”+”.
3. Select Create a new rule…
4. Click More options… in the bottom of the pop-up window.
5. Name the rule->select The recipient… under *Apply this rule if…->is external/internal.
6. Select Outside the organization->OK.
7. Select Block the message… under *Do the following…->reject the message and include an explanation->type in some contents, such as” Do Not Send Emails to OUTSIDE”->Save.

In this way, all users in your organization cannot send emails to external domains any more.

Best regards,
Colin Yao