This is a helpful article on using the NDR text to create the X.500 address, though I found its instructions not quite right.
In my case, I found a slight adjustment needed to make it work.
Let's say you're getting an NDR that says:
For the user jsmith, create a new Exchange email address with a type of X500 (no period in the type - it is X500 not X.500), and enter the value with adjustments as suggested by MS in the article above:
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 "@mgd.domain.com" string.
As an example, I turned this:
/o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=e0c06d4eee7e4ec8b8a38d105ca7793c-joe
In the case above, I moved an Exchange mailbox from one account to another (via exporting the old mailbox to PST and importing into a new mailbox on a new account).