0    Unknown
2    Email Send Error
10    Email address is invalid or not active, no such mailbox exists
20    General Soft Bounce
21    Soft Bounce – DNS Failure
22    Mailbox Full
23    Message Too Large for mailbox recipient
24    Response timed out from destination mail server
25    Message Quarantined
26    Message was flagged due to characteristics of spam
27    Firewall issues on the desitnation mail server prevented this message from being delivered
28    Mail server refused connection
30    Email address is invalid or not active, no such mailbox exists
40    Destination mail service problems
50    Message was flagged due to characteristics of spam
51    Spam
52    Message was flagged due to characteristics of spam (Content)
53    Mail blocked due to attachment
54    Destination mail server does not allow relays, mailbox is most likely incorrect
55    Mail server requires more information on senders domain
60    Auto Reply response detected (e.g. out of office)
110    Mail server requires confirmation
501    rejected due to configuration issues with remote host
502    bounced due to bad connection issues with remote host
503    refused or blocked due to content related reasons
504    bounced due to not being delivered before the bounce-after
505    bounces due to receiving no response from remote host after connecting
506    rejected due to other reasons
507    refused or blocked due to general policy reasons
508    rejected due to SMTP protocol syntax or sequence errors
509    rejected or blocked due to mailbox quota issues
510    refused or blocked due to remote mail server relaying issues
511    bounced due to mail routing issues for recipient domain
512    refused or blocked due to spam related reasons
513    refused or blocked due to virus related reasons
514    Invalid Sender, might require pre-authorization
550    General Send Error
1000    bounced due to invalid or non-existing domains
1001    rejected due to expired, inactive, or disabled recipient addresses
1002    rejected due to bad, invalid, or non-existent recipient addresses
1100    rejected due to bad, invalid, or non-existent recipient addresses