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Office365: How mail is preserved during email migration

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Cut-over Migration

**(Email delay is expected only in Cut-Over migration) **

  • Once the status of your migration batch(es) is shown as “Synced”, the initial synchronization is completed, and deltas will continue to run every 24 hours. In fact, you can start cutting-over (changing MX record) at any given time.  
  • Once you changed the MX record at Public DNS server(s), it's expected that the DNS records MX, A, CNAME, TXT, etc propagation will take from 30 minutes to a few hours based on service providers. 
  • During the DNS propagation time, external emails are still delivered to your On-Prem mail server(s). However, they will be synced back to Office365 mailboxes in the next 24 hours. dd
  • DNS records are completely propagated, external emails will be delivered to your Office365 mailboxes.
  • So there is NO EMAIL LOST.

Staged Migration

  • Once you changed the MX record at Public DNS server(s), it's expected that the DNS records MX, A, CNAME, TXT, etc propagation will take from 30 minutes to a few hours based on service providers. 
  • During the DNS propagation time, external emails are still delivered to your On-Prem mail server(s), however, they will be forwarded to Office365 mailboxes immediately. 
  • DNS records are completely propagated, external emails will be delivered to your Office365 mailboxes.
  • So there is NO EMAIL LOST.

Hybrid Migration

  • Once you changed the MX record at Public DNS server(s), it's expected that the DNS records MX, A, CNAME, TXT, etc propagation will take from 30 minutes to a few hours based on service providers. 
  • During the DNS propagation time, external emails are still delivered to your On-Prem mail server(s), however, they will be **delivered/relayed through On-Prem Hybrid Exchange server to Office365 mailboxes immediately based on "Target Address" AD attributes **( that the target mailbox(es) is in Office365).
  • DNS records are completely propagated, external emails will be delivered to your Office365 mailboxes.
  • So there is NO EMAIL LOST.