Each recipient address for a mail message is added one-by-one to an internal list of recipients. The -d25.1 (a.k.a. -d25) debugging switch causes sendmail to print each address as it is added to this list:

sendto: list
   ctladdr= output of printaddr( ) here (Section 16.3)

After each is added, those that have selected a delivery agent with the F=A (F=A) and F=w (F=w) flags set are further processed by aliasing and by reading the user's ~/.forward file. Each new address that results from this processing is added to the list, and any duplicates are discarded.