Introducing Exchange Online Tenant Outbound Email Limits

Stumbled on this very important gotcha related to TERRL: in the scenario of a large-scale tenant to tenant migration (talking about a tenant with 1000s of users here in a very complex migration), a common practice is to migrate users in waves and use forwarding between tenants to conserve mail flow, both between 'internal' recipients and from external senders, at first in the direction target tenant>source tenant while mailboxes are being synced, then gradually in the other direction as users are being migrated.

Since TERRL counts against any external recipient, this means any message that crosses the tenant boundary is counted against the quota.

Only after all recipients are migrated and no dependency on the source domain(s) is left on the source tenant can the domain names be transferred to the target tenant. At some point in the process, most recipients will be migrated (and licenses for migrated users will no longer be renewed), but all incoming mails to the old tenant will be forwarded to the new tenant (and counted against the quota).

Although tenants with >500 recipients are still paused currently, and quota is pretty high, in some scenario's this will impact delivery at some point near to the actual switchover.

We've already had a case where quota dropped to 5k immediately after MS disabled licenses (too soon), and the forwarding caused between 6-7k external messages per 24h (and that's still in the early stages of migration).

Is there any way to prevent this behavior? It would be nice if there would be a way to contact support to temporarily disable TERRL (just like during a migration you can temporarily turn off EWS throttling to speed up mailbox migration), or Exchange would at least implement some grace period or gradual decrease if licenses are disabled (instead of just dropping the quota immediately from e.g. 500k to 5k).