To anyone impacted by this. Here are a few things you might want to check, other than the obvious user mailbox migration. 0. Set your MX record TTLs as low as

To anyone impacted by this. Here are a few things you might want to check, other than the obvious user mailbox migration. 0. Set your MX record TTLs as low as possible. Even if you aren't ready to change over yet, it will lower propagation delay when you are ready to cut over. 1. Lower your DMARC record TTL, and consider changing it to report only (p=none). If you keep it strict (P=reject), be sure to check your reports for rejections after any changes. 2. Aliases in rackspace aren't in the mailbox list export. This isn't just Bill instead of William, but also info@domain.com or helpdesk@domain.com etc... 3. Allow lists/block lists can be exported from rackspace, and imported in Microsoft but consider starting over from zero if you can. Allow lists are risky. 4. If you are using any advanced email threat protection tools such as Barracuda or Mimecast, they can queue email for re-delivery once the new MX is up. My email is in my profile if anyone has specific questions.