Do these steps to create a rule in Microsoft Exchange On-premise:

1. Go to the Exchange Admin Center (EAC).
2. Log in to the EAC as an administrator.
3. Go to Mail flow in the menu on the left.
4. Go to Rules.
5. Click the add button.
6. Click Create a new rule....
7. Enter Zivver: Secure message by sender + Outbound DLP as the name of the rule.
8. At the bottom, click More options....
9. Under *Apply this rule if..., select The sender... and then is external/internal.
10. Select Inside the organization.
11. Click OK.
12. Click add condition.
13. Under And, select The sender... and then domain is.
14. Enter each sending internal domain, and click the add icon.
15. Click OK.
16. Click add condition.
17. Under And, select The subject or body..., and then subject includes any of these words.
18. Enter (?i)\[secure\].
    
    Tip

    The regex (?i) makes the match case-insensitive.
19. Click the add button.
20. Enter (?i)\[secure\s?email\].
21. Click the add button.
22. Enter (?i)secure\s?email.
23. Click the add button.
24. Click OK.
25. Under *Do the following..., select Modify the message properties..., and then set a message header.
26. Click the first *Enter text... link.
27. Enter zivver-relay.
28. Click OK.
29. Click the second *Enter text... link.
30. Enter smart-sender.
31. Click OK.
32. Click add action.
33. Under And, select Redirect the message to..., and then these recipients.
34. Search for and select the contact that was created previously.
35. Click add ->.
36. Click OK.
37. Under Except if..., click add exception.
38. Select A message header..., and then matches these text patterns.
39. Click *Enter text....
40. Enter skip-zivver-relay.
41. Click OK.
42. Click *Enter text patterns....
43. Enter true.
44. Click the add button.
45. Click OK.
46. Under Choose a mode for this rule, select Enforce.
47. Enable the option Stop processing more rules.
48. Enable the option Defer the message if rule processing doesn’t complete.
    If you enable this option, a message will not leave Exchange until it has been checked by this rule. If, for any reason, Exchange temporarily does not check messages for this rule, processing of those messages will be paused.
49. Enter the following text in the Comments section:

Allow users to deliberately send secure messages via Zivver using a specified email property. For example, a keyword (e.g. ‘[secure]’ in the subject line) or standard email attribute (e.g. a Microsoft Purview sensitivity label applied to the email by the user).

50. Click Save.

The mail flow rule is now set. Please ensure that the priority of all existing rules is correct. If the sent message needs to be processed by other rules first, make sure that the rule created for the Encryption Gateway has a lower priority.

Any outbound message that matches the filter will be submitted to the Zivver SMTP Server. If this is not desired, please deactivate the rule.