Reconnected mailbox = #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

Christian Hans
Christian Hans used Ask the Experts™
After reconnecting an accidentally deleted user mailbox in Outlook 2010 SP3, users receive the following error when sending email.

IMCEAEX-       ....        #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

Im sure this has something to do with the x500 or LegacyDN, but how would I get the old x500 and add it back as an alias to the reconnected mailbox, assuming that would fix the issue that is...

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AmitIT Architect
Distinguished Expert 2017

You can find it in bounce back message. Add that dn value into user mailbox properties as X500.
Exchange Engineer
Distinguished Expert 2018
you will need to look at one of the bounced emails with the x500 address and convert that address properly and then add it to the proxy email address list on the mailbox. see the below link to help you convert the spaces, "+" and so forth. once converted add it to the mailbox proxy addresses as an x500 address.
Christian HansUndecided...


To resolve this issue, use the following method.

Create an X500 proxy address for the old LegacyExchangeDN attribute for the user
To create an X500 proxy address for the old LegacyExchangeDN attribute for the user, make the following changes based on the recipient address in the NDR:
Replace any underscore character (_) with a slash character (/).
Replace "+20" with a blank space.
Replace "+28" with an opening parenthesis character.
Replace "+29" with a closing parenthesis character.
Delete the "IMCEAEX-" string.
Delete the "" string.
Add "X500:" at the beginning.
After you make these changes, the proxy address for the example in the "Symptoms" section resembles the following:


Note The most common items will be replaced. However, there may be other symbols in the LegacyExchangeDN attribute that will also be changed from the way that they appear in the NDR. Generally, any character pattern of "+##" must be replaced with the corresponding ASCII symbol.

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