The -d26.1 (a.k.a. -d26) debugging switch causes sendmail to print the addresses of recipients as they are added to the send queue, which is an internal list of addresses that sendmail uses to sort and remove duplicates from the recipient addresses for a mail message.

On entry to the recipient( ) routine, the -d26.1 debugging switch causes sendmail to print the raw address (as it appears before adding it to the send queue):

recipient (level): output of printaddr( ) here  (Section 16.3) 

An address can be the result of alias expansion. Because the process of aliasing (including :include: and .forward files) can be recursive, it is possible to get too many alias expansions. The level shows the number of alias expansions so far. If that number exceeds the value set by the MaxAliasRecursion option (MaxAliasRecursion), sendmail issues this warning:

aliasing/forwarding loop broken (level aliases deep; maximum max)

Next sendmail compares the new address to others that are already in the send queue. If it finds a duplicate, it prints the following message and skips the new address:

addr in sendq: output of printaddr( ) here (Section 16.3)

Here, addr is the duplicate address. Information about that address is produced with the printaddr( ) routine.