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.
