In PHPUnit it's quite possible to completely mock a class that employs a fluent interface without too much heavy lifting.
$mock = $this->getMock('Zend_Mail');
$mock->expects($this->any())
->method(new PHPUnit_Framework_Constraint_IsAnything())
->will($this->returnSelf());
This has the effect of making any method call on your mock object return a reference to itself.