What do the hard/last bounce codes mean?

Email is delivered and acknowledged by Mail Transfer Agent (MTA) software located at both the sender (BombBomb) and receiver (ISPs). Sometimes, the receiver refuses delivery of the email resulting in a bounce. The receiving MTA effectively 'returns' the email and uses a bounce code to report why. Additionally, sometimes the receiver does not give a reason.  

Bounces are either soft or hard. Soft bounces are generally related to the receiver being too busy to handle more email traffic at this time. BombBomb handles these types of bounces automatically by retrying multiple times to deliver the email over a set period of time. If an email soft-bounces too many times, the receiving MTA will eventually send a hard bounce. BombBomb handles hard bounces by placing those email addresses on the Suppression List. Here is a standard list of bounce codes returned by receiving MTA:

Code   Type    Description

5.0.0   Hard    Address does not exist or email tagged as SPAM

5.1.0   Hard    Other address status 

5.1.1   Hard    Bad destination mailbox address

5.1.2   Hard    Bad destination system address

5.1.3   Hard    Bad destination mailbox address syntax

5.1.4   Hard   Destination mailbox address ambiguous

5.1.5   Hard    Destination mailbox address valid

5.1.6   Hard   Mailbox has moved

5.1.7   Hard   Bad sender's mailbox address syntax

5.1.8   Hard   Bad sender's system address

5.2.3   Hard   Message length exceeds administrative limit.

5.2.4   Hard   Mailing list expansion problem

5.3.0   Hard  Other or undefined mail system status

5.3.2   Hard  System not accepting network messages

5.3.3   Hard  System not capable of selected features

5.3.4   Hard  Message too big for system

5.4.0   Hard  Other or undefined network or routing status

5.4.1   Hard  No answer from host

5.4.2   Hard  Bad connection

5.4.3   Hard  Routing server failure

5.4.4   Hard  Unable to route

5.4.6   Hard  Routing loop detected

5.4.7   Hard  Delivery time expired

5.5.0   Hard  Other or undefined protocol status

5.5.1   Hard  Invalid command

5.5.2   Hard  Syntax error

5.5.4   Hard  Invalid command arguments

5.5.5   Hard  Wrong protocol version

5.6.0   Hard  Other or undefined media error

5.6.1   Hard  Media not supported

5.6.2   Hard  Conversion required and prohibited

5.6.3   Hard  Conversion required but not supported

5.6.4   Hard  Conversion with loss performed

5.6.5   Hard  Conversion failed

5.7.0   Hard  Other or undefined security status

5.7.1   Hard  Delivery not authorized, message refused

5.7.2   Hard  Mailing list expansion prohibited

5.7.3   Hard  Security conversion required but not possible

5.7.4   Hard  Security features not supported

5.7.5   Hard  Cryptographic failure

5.7.6   Hard  Cryptographic algorithm not supported

5.7.7   Hard  Message integrity failure