31.9 Pitfalls

• Macros that are given values while sendmail processes mail may not get the value expected. If that happens, careful hand-tracing of rule sets is required to find the fault. For example, the value in $g (see Section 31.10.16, $g) is the result of sender address rewriting and rewriting by the rule set that is specified in the S= equate of the selected delivery agent. Because $g is used to define the From: header line, errors in that line should be traced through errors in the S= equate's rule set.

• Macros can have other macros as their values. The sendmail program expands macros recursively. As a consequence, unintentional loops in macro definitions can cause sendmail to appear to hang and to eventually segmentation fault and core dump.