What is the Email Rewriting feature?
The Email Rewriting feature of Integration Pro allows users in one tenant to send email as if their mailbox had already been migrated to the target. This allows a company to present a unified presence, even if the migration project takes several months to complete. Or the reverse is also possible. Users who have already been migrated can still send email as if they had not migrated yet. This allows the “old” presence to be maintained until everyone has been migrated.
What configuration is needed?
The email rewrite service must be configured first. SSL certificates will be required. (See the TLS/SSL topic for more information.) Configuration will include the uploading of SSL certificates, and the registration of DKIM records in DNS so that rewritten emails can be signed. The setup wizard will take you through the necessary steps.
What Office365 Groups are created?
Power365 will create an Office365 group named BT-IntegrationPro-DayOne and a group called BT-IntegrationPro-DayTwo.
- DayOne is for users who have not been migrated yet, but you want it to appear as if they have been.
- DayTwo is for users who have been migrated, but you want it to appear as if they have not yet been migrated.
How do you add users to a group?
Add your users to the appropriate groups to make them eligible for the desired rewrite activity. You can use the Office365 admin console, or Powershell. Just remove them from the group when you want to stop rewriting. As these are cloud-based groups, the change will take effect almost immediately.
What happens at Cutover?
When a mailbox user has been cutover and has been a member of the BT-IntegrationPro-DayOne group, do not remove that user from the group after mailbox cutover. In an Premium Integration project type, when an external or internal source user replies to an old message with the original source email address it will route into the source tenant for ultimate delivery over to the target tenant mailbox (before Domain Cutover). The membership of the “Day 1” group should remain populated with the cutover user even after mailbox cutover because it will need to be rewritten before routing to the target tenant mailbox. Otherwise, if not rewritten, any reply to that delivered message would contain the source address, rather than the new target address. This, in turn, could cause end-user frustration and confusion.
When a user in the BT-Integration Pro-DayTwo group that has been Cutover sends a message or calendar invite to a source user (non-migrated), the "from" address will remain unchanged. This behavior is different for external recipients; the "from" address will be rewritten.
This behavior is as designed, due to coexistence constraints related to free/busy services.
One of the primary goals of a Power365 Premium Integration project is to provide coexistence mail and calendaring between the tenants. To achieve this, the target SMTP domain must remain unchanged when sending back to a source (non-migrated) user because when the source user proposes a new time to a calendar meeting, or, when the source user creates a meeting from an existing email, the free/busy would not function if the address had been rewritten.