Our organisation is currently migrating away from a third-party hosted Exchange 2013 server to Office 365. Our organisation spans over multiple branch offices, each with its own domain, and we are migrating users from one office/domain at a time (phased migration, not cut-over).

Due to this, I am not using DirSync, and this involves the two Exchange systems coexisting, until the end of the migration.

The migration has been proceeding relatively smoothly. I have:

  • Created users on the Office 365 platform, with the appropriate SMTP aliases.
  • Used MigrationWiz to migrate our mailbox data.
  • Left the e-mail accounts on the legacy server for the time being and redirected the incoming e-mails to the relevant .onmicrosoft.com alias for each user, so that e-mails sent by internal users still on the legacy platform would be received by the users having migrated to Office 365 already.

This is nevertheless becoming expensive as our legacy platform is paid by mailbox (not an on-premises Exchange server). So we are basically paying twice for the e-mail accounts we have migrated.

If we delete e-mail accounts on the legacy platform, we are getting NDR errors messages, as the X.500 records disappear.

I have got a list of all the X.500 addresses on the legacy platform from the provider.

Some websites ( https://www.experts-exchange.com/questions/28537964/Solving-the-X-500-Issue-with-inter-tenancy-migration-of-Office365.html ) suggest that adding the X.500 address on the Office 365 end would be enough... but I can not get around to how it would work, as how would the legacy platform know that the X.500 for that account is now handled by Office 365? (as X.500, to my knowledge, does not rely on MX records?).

The workaround that I was considering was deleting mailboxes on the legacy platforms, and adding them back as contact, adding the original X.500 address, and then forwarding e-mails sent to that contact to the .onmicrosoft.com address. However, the legacy's management console does not allow this. We do not have access to AD or PowerShell.

In short, I am looking for the best way to be able to delete e-mail accounts on the legacy platform, without causing NDR errors due to X.500 addresses, keeping in mind that we do not have AD or PowerShell access on the legacy platform, and only to a web-based administration platform (Odin).

Thank you!

1 Answer 1


The X.500 solution is used when all mailboxes are on to the new platform (whether Office365 or a self hosted). It allows end users to send email to the old addresses and they are delivered. The key bit being that both the sender and the recipient are on the same platform.

In your scenario, it isn't going to work. Contacts would be the method you could use, putting the external onmicrosoft.com address on to the contact, as well as the X.500 address. However if the management tools do not allow that then you are pretty much stuck with what you are doing now, and taking the cost hit. The only other solution is to tell users that the nicknames file is no longer valid and they need to select the recipient fresh.

All if your options for making it easier for the end users are dependant on full access to the old platform, which you don't have.

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