Issue and Resolution : Mail flow NDR issue after Mailbox Migrated to EXO - '550 5.1.11 Legacy encapsulated email address lookup'

 Hello All

Here is the one of the issue which i faced and fixed for the user who migrated to exchange online from exchange 2016 onprem Servers

Issue 

After mailbox migration , When sending an email to the old cached address, users will get a bounce back  NDR error. Given Below

Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'

The reason was the legacy X500 address was missing and we need to add legacy x500 address from the NDR Error

And now the question is: Why?

Mailbox exists , got a valid SMTP address, emails from other senders are delivered without any problems …

The trail leads to Outlook and auto completion of addresses. In the cache of sender’s Outlook  is stored a old recipient address. When addressing a message, Outlook automatically suggests the address to which was  previously sent messages.

For a internal addresses Outlook uses LegacyExchangeDN, and these addresses are stored in Outlook cache. And our recipient has the same SMTP address , but different LegacyExchangeDN

The easiest solution is to delete an old entry from Outlook suggested contacts. Re-selecting recipient  from GAL or manual typing address will save correct LegacyExchangeDN to Outlook cache. And the problem will be solved.

Well, yes but this solution is a bit annoying for users. and, not really works well with a larger number of affected senders.

Second option is to add old legacyExchangeDN as a X500 address 

Resolution

Powershell Sample to convert the IMCEAX NDR to X500 Value from NDR

$x = "enter IMCEAEX full value" $x = $x -replace "IMCEAEX-_", "X500:/" -replace "\+20", " " -replace "\+28", "(" -replace "\+29", ")" -replace "_", "/" -replace "\+2E","." -replace '@.*'

The easiest way to convert is use Windows Poweshell ISE

Open the windows powershell IS from windows 10 or 11

Add the X500 address to the end user email address from Onprem and wait for Hybrid  Sync to compete. 

Usually X500 takes few hours to update

Now the end  users are able to send the email from old cache.

I hope this blog helps to understand to fix the X500 related to issue after mailbox migration . Happy Learning 😃