During a phased tenant-to-tenant migration, some of your users move to the new tenant before others. Turn on Coexistence in Copy options to keep mail flowing correctly during that transition: users who've already migrated can still be reached at their old address, and their outgoing mail can carry an optional note explaining the change.
Note: This release supports cloud-to-cloud Microsoft 365 migrations only, with one-way forwarding (source to target). See Limitations below for what's not included yet.
How-to
You set up coexistence in Copy options, as part of configuring your mailbox migration.
Start a Copy mailboxes migration and follow the steps until you reach Copy options.
Select the Coexistence tab.
Turn on Coexistence.
To add a note to outgoing mail from migrated users, check Outbound disclaimer and enter your text.
Finish setting up your migration and start the copy.
How it works
When you turn on Coexistence for a mailbox, ShareGate Migrate automatically configures two things in Exchange Online once that mailbox migrates:
A forwarding rule that redirects mail sent to the user's old address to their new mailbox, so nothing bounces.
An anti-spam exception so the forwarded mail isn't mistaken for spam by your tenant's default Microsoft 365 policy, the most common reason forwarding silently fails.
Note: Coexistence relies on native Exchange Online rules that ShareGate Migrate sets up for you. Don't manually recreate or edit these rules in the Exchange admin center or via PowerShell. Doing so can break forwarding and cause ShareGate Migrate to lose track of your coexistence setup.
Outbound disclaimer (optional)
Check this box to add a short note to outgoing mail from migrated users during the coexistence window, telling external contacts why they're now receiving mail from a new domain.
The disclaimer supports HTML formatting, such as bold text and links.
It also supports personalization tokens like
%%DisplayName%%, which insert the sender's name automatically.
Note: Image and script tags aren't supported. Exchange Online strips or rejects them.
For the full list of supported tokens and formatting rules, see Microsoft's documentation on mail flow rule disclaimers, signatures, footers, and headers.
Limitations
Forwarding is one-directional only (source to target) in this release. Forwarding replies back to the source is planned for a later release.
Not-yet-migrated users aren't visible in the target tenant's address book yet.
Cross-tenant calendar availability (free/busy) isn't available yet.
Outbound address rewrite isn't offered. Mail sent by migrated users shows their new address.
Hybrid or on-premises Exchange isn't supported. Both tenants must be cloud-based Microsoft 365.