The -d20.1 (a.k.a. -d20) debugging switch causes sendmail to print each recipient address before it is rewritten by the canonify rule set 3 and the parse rule set 0:

--parseaddr(addr)

Here, addr is the recipient address before it is rewritten and before any aliasing has been performed on it.

The -d20.1 debugging switch also causes sendmail to print information about problems that might exist in recipient addresses. If an address contains any control or whitespace character that is not an isspace(3) character, sendmail prints the following message and skips that address:

parseaddr-->bad address

If an address is empty (that is, if it is composed entirely of an RFC2822-style comment), sendmail prints the following and skips that address:

parseaddr-->NULL

After the recipient address has been rewritten by the canonify rule set 3 and the parse rule set 0, and if a delivery agent was successfully selected, sendmail prints the result using the printaddr( ) routine.