General overview of the mail processing chain

A generic (yet quite common) mail processing chain.

  ------------		        managed by      	        ---------------- 	
 |   mua      |<-----------------------------------------------| user mailbox	|	
 |(e.g. mutt) |       	       	       	       	       	       |(e.g. ~/Maildir)|	
  ------------		 				      	----------------	
       |  		       	       	       	       	     	 ^         ^		
       |  compose the message and send   		      	 |     	   |		
       |  it through a pipe	       	  		      	 |     	   |		
       | 						      	 |     	   |	       	       	       
       v 						      	 |     	   |	    -------------------
  -------------						      	 |     	   |	   |  external filter  |
 | SMTP client |					      	 |     	   |	   |(e.g. spamassassin)|    
  -------------        	       	       	       	 	      	 |     	   |	    -------------------
       |       					 	      	 |     	   |	       |       	   ^
       |       					 	      	 |     	   |	       |	   |
       |       	       	       	       	       	       	         |         |   	       |       	   |
       |            -------------------	      file transport     |     	   |	       v       	   |
       `---------> | SMTP local server |-------------------------       ---------------------------------   
       ,---------> |     (e.g. exim)   |      pipe transport   	      |  mail delivery/processing agent |
       |           |   	       	       |----------------------------->|        (e.g. procmail)       	|       	     
       |       	   |   	       	       |-------                        ---------------------------------
       |            -------------------	       |  	       	     			
       |			       	       |  	       	     			
       |			       	       |       	       	     			
       |	  		       	       |       	       	       	       	       	
  ------------------------     	       	       |       	       	 	  		
 |  pop retrieval program |	       	       | SMTP transport	 
 |  (e.g. fetchmail)      |		       |       	   	 
  ------------------------		       |       	   
       ^				       |
       |				       |
       |     	       	       	       	       |      
       |				       v      
  -------------------             -------------------- 
 | remote pop/imap   |		 | remote SMTP server |
 |      server       |           |                    |
  -------------------		  --------------------