The -d21.1 (a.k.a. -d21) debugging switch causes sendmail to print each step that it takes in rewriting addresses with rules. The -d21.1 debugging switch causes output to be produced that is identical to the output produced by the -bt command-line switch (Section 8.1):

rewrite: rule set name or number      input: address
rewrite: rule set name or number    returns: address

First, the address (workspace) is displayed for the rule set whose name or number is shown before rewriting, and second, the address is shown after rewriting.

Because rules are recursive by nature, they can sometimes cause infinite loops (Section 18.7.2). When a rule loops more than 100 times, the following error is issued:

Infinite loop in rule set name or number, rule rule number

If the -d21.1 debugging switch was also invoked, the preceding error is followed by:

workspace: state of rewritten address, so far, is shown here