Original message:
                   
                   To: bill@irs.gov
                   Hi.
                   
qmail-inject    Fill in the complete envelope and header:
                   
     |             (envelope) from joe@heaven.af.mil to bill@irs.gov
     |             From: joe@heaven.af.mil
     |             To: bill@irs.gov
     |             
     |             Hi.
     V             
                   
qmail-queue     Store message safely on disk.
                Trigger qmail-send.
     |             
     V             
                   
qmail-send      Look at envelope recipient, bill@irs.gov.
     |          Is irs.gov in locals? No.
     |          Is bill@irs.gov in virtualdomains? No.
     |          Is irs.gov in virtualdomains? No.
     |          Is .gov in virtualdomains? No.
     |          Deliver remotely to bill@irs.gov.
     V          
                   
qmail-rspawn    Run qmail-remote.

     |             
     V             

qmail-remote    Look at host name, irs.gov.
                Is irs.gov listed in smtproutes? Yes: ":bigbang.af.mil".
                Look up DNS A for bigbang.af.mil and connect by SMTP:
                   
                   MAIL FROM:<joe@heaven.af.mil>
                   RCPT TO:<bill@irs.gov>