From the Facebook Postmaster page. These are the reply codes that Facebook mailservers can send to a connecting server.
Our MTA's are set to take necessary actions on specific SMTP response codes in order to maintain our reputation !

Facebook System Resource Issues

These codes indicate a temporary issue internal to Facebook's system. Administrators observing these issues are not required to take any action to correct them.
INT-T1
INT-T2
INT-T3

INT-T4
INT-T5
INT-T6
INT-T7
INT-T8
INT-T9
INT-T10
INT-T11
INT-T12

Sender Domain Issues

These codes indicate an issue with either your domain's DNS record or your mail server's reverse DNS record.

Permanent Issues

These will persist until action is taken to correct them.
DNS-P1  Your SMTP MAIL FROM domain does not exist.
DNS-P2  Your SMTP MAIL FROM domain does not have an MX record.
DNS-P3  Your mail server does not have a reverse DNS record.

Temporary Issues

These may only persist for a short time. Administrators may want to investigate them further if they occur often.
DNS-T1  Your SMTP MAIL FROM domain exists but does not currently resolve.
DNS-T2  You mail server's reverse DNS record does not currently resolve.

Recipient Issues

These codes indicate an issue with one of the message recipients. Administrators observing these issues are not required to take any action to correct them.
RCP-P1  The attempted recipient address does not exist.
RCP-P2  The attempted recipient's preferences prevent messages from being delivered.
RCP-T1  The attempted recipient address is not currently available due to an internal system issue. This is a temporary condition.
INT-P1  The attempted recipient address does not exist.
INT-P3  The attempted recpient group address does not exist.
INT-P4  The attempted recipient address does not exist.

Message Issues

These codes indicate an issue with the specific message that triggered them. Message authors observing these issues are required to take action to correct them.
MSG-P1  The message exceeds Facebook's maximum allowed size.
INT-P2  The message exceeds Facebook's maximum allowed size.
MSG-P2  The message contains an attachment type that Facebook does not accept.
MSG-T1  The number of recipients on the message exceeds Facebook's allowed maximum.

Connection Limits

These codes indicate an issue with the connections from your mail servers. They are temporary conditions and administrators observing them are not required to take any action to correct them.
CON-T1  Facebook's mail server currently has too many connections open to allow another one.
CON-T2  Your mail server has opened too many new connections to Facebook's mail servers in a short period of time.
CON-T3  Your mail server currently has too many connections open to Facebook's mail servers.
CON-T4  Your mail server has exceeded the maximum number of recipients for its current connection.

Policy Violations

These codes indicate that either the message or your mail server has violated Facebook's abuse prevention policies.

Permanent Mail Server Policies

These will persist until the administrator takes action to correct them.
POL-P1  Your mail server's IP Address is listed on the Spamhaus PBL.
POL-P2  Facebook will no longer accept mail from your mail server's IP Address.

Permanent Message Policies

Message authors observing these issues are required to take action to correct them.
POL-P5  The message contains a virus.
POL-P6  The message contains a url that has been blocked by Facebook.

Permanent Domain Policies

These will persist until action is taken to correct them.
POL-P7  The message does not comply with Facebook's Domain Authentication requirements.

Temporary Mail Server Policies

These will expire after the stated duration. Administrators are encouraged to take corrective action to prevent them from reoccurring.
POL-P3  Facebook is not accepting messages from your mail server. This will persist for 4 to 8 hours.
POL-P4  Facebook is not accepting messages from your mail server. This will persist for 24 to 48 hours.
POL-T1  Facebook is not accepting messages from your mail server, but they may be retried later. This will persist for 1 to 2 hours.
POL-T2  Facebook is not accepting messages from your mail server, but they may be retried later. This will persist for 4 to 8 hours.
POL-T3  Facebook is not accepting messages from your mail server, but they may be retried later. This will persist for 24 to 48 hours.