This problem is not common but quite annoying and usually hard to understood by users.
In most cases problem occurs If you remove mailbox and re-create it with the same AD account , for an example after firing and re-hiring of the employee after some time.
It also occurs after migrations.
When user sends an email to a recipient, to which earlier messages was delivered without any problem. Sudennly gets an Non Delivery Report with this content:
Delivery has failed to these recipients or distribution lists:: Test User The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.. Diagnostic information for administrators: Generating server: server01.exchange IMCEAEX_O=Company_OU=EXCHANGE+20ADMINISTRATIVE+20GROUP+20+28FYDIBOHF23SPDLT29_CN=RECIPIENTS_CN=user@xxx.xx #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##
And now the question is: Why?
English 
polski