Local to local

        Original message:
                   
                   To: fred
                   Hi.
                   
qmail-inject    Fill in the complete envelope and header:
                   
     |             (envelope) from joe@heaven.af.mil to fred@heaven.af.mil
     |             From: joe@heaven.af.mil
     |             To: fred@heaven.af.mil
     |             
     |             Hi.
     V             
                   
qmail-queue     Store message safely on disk.
                Trigger qmail-send.
     |             
     V             
                   
qmail-send      Look at envelope recipient, fred@heaven.af.mil.
     |          Is heaven.af.mil in locals? Yes.
     |          Deliver locally to fred@heaven.af.mil.
     V          
                   
qmail-lspawn ./Mailbox
                   
     |          Look at mailbox name, fred.
     |          Is fred listed in qmail-users? No.
     |          Is there a fred account? Yes.
     |          Is fred's uid nonzero? Yes.
     |          Is ~fred visible to the qmailp user? Yes.
     |          Is ~fred owned by fred? Yes.
     |          Give control of the message to fred.
     |          Run qmail-local.
     V          
                   
qmail-local fred ~fred fred '' '' heaven.af.mil joe@heaven.af.mil ./Mailbox
                   
                Does ~fred/.qmail exist? No.
                Write message to ./Mailbox in mbox format.

Local to local extension

          Original message:
                   
                   To: fred-sos
                   Hi.
                   
qmail-inject    Fill in the complete envelope and header:
                   
     |             (envelope) from joe@heaven.af.mil to fred-sos@heaven.af.mil
     |             From: joe@heaven.af.mil
     |             To: fred-sos@heaven.af.mil
     |             
     |             Hi.
     V             
                   
qmail-queue     Store message safely on disk.
                Trigger qmail-send.
     |             
     V             
                   
qmail-send      Look at envelope recipient, fred-sos@heaven.af.mil.
     |          Is heaven.af.mil in locals? Yes.
     |          Deliver locally to fred-sos@heaven.af.mil.
     V          
                   
qmail-lspawn ./Mailbox
                   
     |          Look at mailbox name, fred-sos.
     |          Is fred-sos listed in qmail-users? No.
     |          Is there a fred-sos account? No.
     |          Is there a fred account? Yes.
     |          Is fred's uid nonzero? Yes.
     |          Is ~fred visible to the qmailp user? Yes.
     |          Is ~fred owned by fred? Yes.
     |          Give control of the message to fred.
     |          Run qmail-local.
     V          
                   
qmail-local fred ~fred fred-sos - sos heaven.af.mil joe@heaven.af.mil ./Mailbox
                   
                Does ~fred/.qmail-sos exist? Yes: "./Extramail".
                Write message to ./Extramail in mbox format.

Local to virtual

      Original message:
                   
                   To: dude@tommy.gov
                   Hi.
                   
qmail-inject    Fill in the complete envelope and header:
                   
     |             (envelope) from joe@heaven.af.mil to dude@tommy.gov
     |             From: joe@heaven.af.mil
     |             To: dude@tommy.gov
     |             
     |             Hi.
     V             
                   
qmail-queue     Store message safely on disk.
                Trigger qmail-send.
     |             
     V             
                   
qmail-send      Look at envelope recipient, dude@tommy.gov.
     |          Is tommy.gov in locals? No.
     |          Is dude@tommy.gov in virtualdomains? No.
     |          Is tommy.gov in virtualdomains? Yes: "tommy.gov:fred".
     |          Deliver locally to fred-dude@tommy.gov.
     V          
                   
qmail-lspawn ./Mailbox
                   
     |          Look at mailbox name, fred-dude.
     |          Is fred-dude listed in qmail-users? No.
     |          Is there a fred-dude account? No.
     |          Is there a fred account? Yes.
     |          Is fred's uid nonzero? Yes.
     |          Is ~fred visible to the qmailp user? Yes.
     |          Is ~fred owned by fred? Yes.
     |          Give control of the message to fred.
     |          Run qmail-local.
     V          
                   
qmail-local fred ~fred fred-dude - dude tommy.gov joe@heaven.af.mil ./Mailbox
                   
                Does ~fred/.qmail-dude exist? No.
                Does ~fred/.qmail-default exist? Yes: "./Mail.tommy".
                Write message to ./Mail.tommy in mbox format.

Local to local alias

      Original message:
                   
                   To: help
                   Hi.
                   
qmail-inject    Fill in the complete envelope and header:
                   
     |             (envelope) from joe@heaven.af.mil to help@heaven.af.mil
     |             From: joe@heaven.af.mil
     |             To: help@heaven.af.mil
     |             
     |             Hi.
     V             
                   
qmail-queue     Store message safely on disk.
                Trigger qmail-send.
     |             
     V             
                   
qmail-send      Look at envelope recipient, help@heaven.af.mil.
     |          Is heaven.af.mil in locals? Yes.
     |          Deliver locally to help@heaven.af.mil.
     V          
                   
qmail-lspawn ./Mailbox
                   
     |          Look at mailbox name, help.
     |          Is help listed in qmail-users? No.
     |          Is there a help account? No.
     |          Give control of the message to alias.
     |          Run qmail-local.
     V          
                   
qmail-local alias ~alias help - help heaven.af.mil joe@heaven.af.mil ./Mailbox
                   
		Does ~alias/.qmail-help exist? Yes: "john".
		Forward message to john.

Local to remote

 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? No.
		Look up DNS MX/A for irs.gov and connect to it by SMTP:
                   
                   MAIL FROM:<joe@heaven.af.mil>
                   RCPT TO:<bill@irs.gov>

Null client

    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>

qmail-smtpd     Receive message by SMTP from another host:
                   
     |             MAIL FROM:<bill@irs.gov>
     |             RCPT TO:<joe@heaven.af.mil>
     |          
     |          Is $RELAYCLIENT set? No.
     |          Is heaven.af.mil in rcpthosts? Yes.
     |          Accept RCPT.  
     V             
                   
qmail-queue     Store message safely on disk.
                Trigger qmail-send.
     |             
     V             
                   
qmail-send      Look at envelope recipient, joe@heaven.af.mil.
     |          Is heaven.af.mil in locals? Yes.
     |          Deliver locally to joe@heaven.af.mil.
     V          
                   
qmail-lspawn ./Mailbox
                   
     |          Look at mailbox name, joe.
     |          Is joe listed in qmail-users? No.
     |          Is there a joe account? Yes.
     |          Is joe's uid nonzero? Yes.
     |          Is ~joe visible to the qmailp user? Yes.
     |          Is ~joe owned by joe? Yes.
     |          Give control of the message to joe.
     |          Run qmail-local.
     V          
                   
qmail-local joe ~joe joe '' '' heaven.af.mil bill@irs.gov ./Mailbox
                   
                Does ~joe/.qmail exist? No.
                Write message to ./Mailbox in mbox format.

qmail-smtpd     Receive message by SMTP from another host:
                   
                   MAIL FROM:<spammer@aol.com>
                   RCPT TO:<bill@irs.gov>
                
                Is $RELAYCLIENT set? No.
                Is irs.gov in rcpthosts? No.
                Reject RCPT.

qmail-smtpd     Receive message by SMTP from another host:
                   
     |             MAIL FROM:<joe@heaven.af.mil>
     |             RCPT TO:<bill@irs.gov>
     |          
     |          Is $RELAYCLIENT set? Yes: "".
     |          Accept RCPT.  
     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? No.
		Look up DNS MX/A for irs.gov and connect to it by SMTP:
                   
                   MAIL FROM:<joe@heaven.af.mil>
                   RCPT TO:<bill@irs.gov>

출처: http://cr.yp.to/qmail/pictures.html