Deliverability of e-mail is critical to having a healthy list and on-going campaign. This list will help you discect why your emails are getting bounced.

Block - when the e-mail can’t get through due to filters or user generated complaints
1001 Spam Complaint
1002 Blacklist
1003 ISP Block
1004 Content Block
1005 URL Block
1006 Excess Traffic
1007 Security Violation/Virus
1008 Open Relay
1009 List Management
1010 Authentication
1999 Other

Hard Bounce - when the e-mail can’t get through due to account errors.
2001 Unknown User
2002 Bad Domain
2003 Address Error
2004 Closed Account
2999 Other

Soft Bounce - when the e-mail can’t get through due to account issues.
3001 Mailbox Full
3002 Inactive/Disabled Account
3003 Greylist
3999 Other

Technical - when the e-mail can’t get through due to non-user or non-account issues
4001 Sever Too Busy
4002 Data Format Error
4003 Network Error
4004 Other Receiver Error
4999 Other