The secret about a successful export-to-pst and import-from-pst migration is called Add Custom Address, type X500

The last few months, I have had the below question pop up regularly…


is it possible to export the content of my On Premises mailboxes to PSTs, and then import them into my brand new Exchange Online environment?


Yes, you definitely export your mailbox content to a PST, and import that PST back into your new mailbox

But, the problem that usually pops up is, that when replying to a mail that has been imported back into the new mailbox from the PST, and it’s a reply to a another mailbox homed in the same mail environment, a user gets the following NDR:

Delivery has failed to these recipients or groups, The e-mail address you entered couldn’t be found…. as seen below:



One way of solving this, is when you reply to a mail imported from your PST, and destined for anyone in the same Exchange environment, to just remove the name, and search for it again in the Global Address List, that will work

But: is that user friendly?

Definitely not, so, time to investigate, and come up for a better way to solve this!

Setting the scene

I currently have an Exchange 2010 Sp1 On Premises deployed, with two mailbox-enabled users:


Both mailbox-enabled users have sent a few mails between one-another:


Now, I want to export the content of these two mailboxes to a PST, that I want to import into a brand new Exchange Online environment. First I will delegate myself (being the administrator) the abililty to export mailboxes to a PST using the Exchange Management Shell cmdlet New-MailboxExportRequest, by running the following line in the EMS: New-ManagementRoleAssignment –role “Mailbox Import Export” –user “MsBelgium\Administrator”


Followed by the export itself:, issuing New-MailboxExportRequest:


And double-check to make sure it’s finished, using Get-MailboxExportRequest:


Next up: Create two mailbox-enabled users in my Office 365 environment, and hand out the same e-mail addresses to both users:



Everything is looking as it should, being, ok Smile

Time to open up Robin’s mailbox using Outlook Anywhere, and import the previously exported PST file:




Everything seems to be in order, but now, Robin wants to reply to a mail received from his colleague Rita, now watch the MailTip provided by Outlook 20101


But let’s just ignore the warning provided by the MailTip, and press Send…. and let’s have a look at the NDR we are getting immediately:


and the below info, Diagnostic information for administrators:


Interesting, searching the web, got me more information about this issue, for example at the following links:

So, how to solve this?

Simple: Use the Exchange Management Shell, or the Exchange Management Console to add the X500 address to the mailbox-enabled users



After clicking OK, and checking with the Shell we can see the X500 address has been successfully added:


When replying to the same message, we don’t get the MailTip anymore Smile


And the message arrives, without any problem:


Exchange simply rocks!