Hello, @Thirza Natasha,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you want to send internal emails to a third-party email security gateway for scanning, and then send the emails deemed safe by the gateway scanning to recipients in the same domain.
As you said, you cannot scan directly with a third-party email security gateway during the transmission of internal emails. This is because there is no need to use a third party for internal emails to be secure, and also the internal connector cannot be changed. If you wish to use the purchased third-party email security gateway, you can consider the alternatives given below.
1.Create a mail flow rule using the Send messages to a moderator template according to the settings below. The obscured portion is a transitory mailbox that temporarily receives outgoing mails, and the owner of this mailbox has the right to release them or not.
2.Enable the new rule.
3.The sender sends an email to a user on the same domain, the transition mailbox receives a notification of whether or not to party line this email, and the receiver receives the email only if the owner of the transition mailbox selects Approve, at which point you can use a third-party gateway to scan this mailbox.
What I have provided above is just an alternative that may be able to help you, one that will take the mailbox owner's time and will have problems with mail delays. It is up to you to decide whether to take this option or not.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang