How to test that correct email messages were sent?

[Andrey Botalov <bo...@gmail.com>]

I have a performance test after execution of which some email messages 
are sent to user's email. I'd want to verify that correct messages were 
sent. Buy correctness I mean that they have correct|To:|and 
|Subject:|headers

I added Mail Reader Sampler at the end of the plan. At the time of its 
execution I have variables 
|mail_1_subject|,|mail_2_subject_1|,|mail_2_subject_2|, 
...,|mail_2_subject_N|set where N is stored in another 
variable|mail_2_N|. I'd want to verify that sub-sample emails have 
correct subjects. Subsamples may be received in another order than those 
variables are set so I should search those sub-samples for presence of 
those subjects.

I thought that I should add some Regular Expression Extractors each of 
which with regex like|Subject: ${mail_2_subject_X}|and negative 
reference counter. But I know number of messages to be sent only at 
execution time, not at writing JMX stage. So I can't add those Regular 
Expression Extractors.

So how can verify that correct messages were sent?