How can I see the reasons for the bounces on undeliverable emails?

You will receive the bounces to the address of the sender, the reasons are under 'Diagnostic-Code'.
 
Typical reasons include:
  • 'User unkown' or 'No such recipient' - this means that this address was not found.
  • 'Unable to lookup DNS' or 'Connection timed out' means the domain can not be found.