You monitor the success of your campaigns with online reports and make adjustments accordingly. The measuring of all those opens, click-thru activity and bounces its called tracking.
Only a madman works without any kind of way to grade his or her performance. It would be lunacy to send out email marketing messages without gauging how effective they are.
Of course, a good indication is a general uptick in business activity and that might tell you if the email marketing is working generally, but what of the performance of the individual messages that you send out?
In order to refine your message so that you are sending out marketing messages in the most optimized fashion, you need an effective way to measure:
How many of your emails made it to the inboxes of the individual recipients
Which email addresses are no longer valid
What percentage of your list opened up your emails
Who individually opened up your emails
Who clicks on the links that you provide within your emails
Who forwards your email campaigns to their friends
Who unsubscribes from your list
The performance of one campaign versus another
Use Benchmark Email's included email tracking and reporting
to get this information easily.