When sendmail runs as a daemon, it must disconnect itself from the terminal device that is used to run it. This prevents keyboard signals from killing it and prevents it from hanging (on a dial-in line waiting for carrier detect, for example).

The -d52.1 (a.k.a. -d52) debugging switch shows sendmail disconnecting from the controlling terminal device:

disconnect: In fd Out fd, e=addr  

For both its input and output connections, the fd is a decimal representation of the file descriptor number. The addr is a hexadecimal representation of the address that contains the envelope information. If the LogLevel option (LogLevel) is greater than 71, sendmail syslog(3)s the following message to show that it has disconnected:

in background, pid=pid

Here, pid is the process identification number of the child process (the daemon).