The special address <> is used as the envelope sender when sendmail bounces a mail message. This address is intended to prevent bounced messages from bouncing. Unfortunately, not all configuration files properly handle this form of sender address. The stock SunOS configuration files prior to Solaris 2.3, for example, caused sendmail to enter an endless loop when processing <>. Also, some UUCP implementations get confused when they are executed with command-line arguments of:

-f <>

As an interim measure, until all programs learn to correctly handle the <> address, you can use the F=g delivery agent flag to suppress that address for selected delivery agents. If the F=g delivery agent flag is set for a delivery agent, it uses the value of $g ($g) in place of the <>, where $g contains $n (usually MAILER-DAEMON, $n) with an @ and the local domain name appended.