First of all, there are two reasons for an address to be put in quarantine:
1) delivery error with a clear indication that the address is invalid (either it never existed or it does not exist any more)
2) the address contains a syntax error (no '@', incorrect domain zone, etc.)
For delivery errors:
In file « SendLog.csv », you'll find the field isInvalidMailbox.
SendLog.isInvalidMailbox = 1 means that the address is in Quarantine (in other words it's invalid) following delivery error processing.
For syntax errors:
In file « ContactsList.csv », you'll find the fields isInvalidMailbox and Domain.
If (CotactLIst.isInvalidMailbox = 1) AND (ContactList.Domain is empty) => the address is syntactically incorrect.