diagram

<B>Digital的Unix操作系统VAX 4.2源码</B>

+---------+   +---------+   +---------+
| sender1 |   | sender2 |   | sender3 |
+---------+   +---------+   +---------+
     |  	   |             |
     +----------+  +  +----------+
		|  |  |
		v  v  v
            +-------------+
            |   sendmail  |
            +-------------+
		|  |  |
     +----------+  +  +----------+
     |  	   |             |
     v             v             v
+---------+   +---------+   +---------+
| mailer1 |   | mailer2 |   | mailer3 |
+---------+   +---------+   +---------+