To monitor SMTP connections, check the SMTP log file located at: /var/log/qmail/smtpd. Look for entries having the text tcpsvd: info:. These lines record new SMTP connections and the total number of connections out of the maximum is recorded.

root@hostname:/var/log/qmail/smtpd# tail current 
2007-04-11_04:45:53.84383 tcpsvd: info: end 31982 exit 0
2007-04-11_04:45:53.84389 tcpsvd: info: status 0/20

The above lines show that:

1. an SMTP connection was just closed, and,

2. the number of active connections is 0 out of a maximum of 20.