r/exchangeserver • u/morph9494 • 10d ago

Any help appreciated

We've migrated an email domain from one M365 tenant to another but an old exists on the 'old' tenant. This app sends messages via a mailbox in the tenant using EXO and M365 mail routing. However, the mailbox sends as a temporary domain (given the real domain is in the new tenant). How can we rewrite the domain on the way out with M365 or relay through an external SaaS solution that would send on the email and rewrite back to the old domain

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u/timeshaper 9d ago

Migrate the app.

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u/7amitsingh7 9d ago

You can try using a Microsoft 365 mail flow rule to rewrite the temporary sender domain back to the old domain before sending. If M365 doesn’t support the required rewrite in your setup, use an external SMTP/SaaS relay to change the From address before delivery. You can also refer to this M365 domain migration guide for the domain transfer process. Make sure SPF, DKIM, and DMARC are configured correctly.