In the SMTP RCPT command, sendmail is required to express the recipient's address relative to the local host. For domain addresses, this simply means that the address should be RFC2821-compliant.

The -d12.1 (a.k.a. -d12) debugging switch causes sendmail to print the address as it appeared before it was made relative:

remotename(addr)

If the addr is for the sender or recipient and is being processed from a queue file, nothing more is printed, and the addr is processed by canonify rule set 3. If the delivery agent for the recipient has the F=C flag set (F=C) and the recipient address lacks a domain part, the domain of the sender is appended, and the result is processed by the canonify rule set 3 again. Sender/recipient-specific rule sets are then applied (1 and S= for the sender, or 2 and R= for the recipient). Next, the final rule set 4 is applied, and any sendmail macros in the result are expanded. Finally, the fully qualified and relative address is printed as:

remotename => `addr'