The sendmail program is able to log a wide variety of information about what it is doing. There is no default file for recording information. Instead, sendmail sends all such information via the Unix syslog(3) mechanism. The disposition of messages by syslog is determined by information in the file /etc/syslog.conf (see Section 14.3.2). One common scheme places non-critical messages in /var/log/syslog but routes important messages to /dev/console or /var/adm/messages.
The meaningful values for the logging level, and their syslog priorities, are outlined here. Higher logging levels include the lower logging levels. For example, logging level 2 also causes level 1 messages to be logged.
Minimal logging. See See this section for examples of what is logged at this setting.
Serious system failures and security problems logged at LOG_CRIT or LOG_ALERT.
Communication failures (e.g., lost connections or protocol failures) logged at LOG_CRIT.
Malformed addresses logged at LOG_NOTICE. Transient forward/include errors logged at LOG_ERROR. Connect timeouts logged at LOG_NOTICE.
qffilenames and minor errors logged at LOG_NOTICE. Out-of-date alias databases logged at LOG_INFO. Connection rejections (via libwrap.a or one of the
check_rule sets) logged at LOG_NOTICE.
A record of each message received logged at LOG_INFO. Envelope cloning logged at LOG_INFO.
SMTP VRFY attempts and messages returned to the original sender logged at LOG_INFO. The ETRN and ...