Processing of rules requires that all addresses be divided into tokens. The -d22.1 (a.k.a. -d22) debugging switch causes sendmail to print the various steps it takes in tokenizing an address.

In addition to tokenizing, the prescan( ) routine also normalizes addresses. That is, it removes RFC2822-style comments and recognizes quoted strings. Be aware that rules are also viewed as addresses and processed by prescan( ) when the configuration file is being read.

The -d22.1 debugging switch tells sendmail to complain if the first token in the address it is parsing turns out to be nothing:

prescan: null leading token

This can happen if an address (or rule) contains only RFC2822-style comments in parentheses.