Understanding delivery error messages
Understand and correct the error messages you receive
Error 4XX: Temporary unavailability
Understanding 4XX errors
Error 550 5.1.1: address unreachable
Understanding the 550 error 5.1.1
Error 550 5.7.1: insufficient rights
Understanding error 550 5.7.1
Error 553 : Invalid address
Understanding error 553
Error 554 5.5.1: Empty recipient list
Understanding error 554 5.5.1