When sendmail bounces a message, it notifies the envelope sender of the problem using DSN. A critical part of DSN is the error code, which provides more detail than the usual SMTP error code. A DSN error code, as reported with syslog, looks like this:

dsn=5.7.0

Here, the 5 means it was a permanent error, and the 7 means it was a security or policy rejection. The meanings of DSN status codes are documented in RFC1893.