Besides backup, any IT division should have a disaster recovery plan. You will find a few tips below relating to the development of such a plan and to what issues one should pay special attention in the course of backup planning.
The email address wasn't found at the destination domain. It might be misspelled or it might not exist any longer. Try retyping the address and resending the message.
If that doesn't work, contact the recipient (by phone or instant messaging, for example) to check that the address is correct. If the problem continues, forward this message to your email admin.
For Email Administrators
For more tips to help fix this issue, see DSN 5.1.1 Errors in Exchange Online and Office 365.
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