Header lines (Section 25.1) from the configuration file and from mail messages are processed by the chompheader( ) routine before they are included in any mail message. That routine parses each header line to save critical information, to check for validity, and to replace default values with new values.

The -d31.2 debugging switch^[2] shows that sendmail is about to check whether it should replace a From: or Resent-From: header with the one defined by the H configuration command. If the configuration file is not being read and if sendmail is not processing the queue, the following test is made:

^[2] There is no -d31.1 information.

comparing header from (header) against default (address or name) 

The value of the From: or Resent-From: header is compared to the sender's address and to the sender's name. If it is the same as either one, the address is replaced.