There are options in the control panel to change the way subscriptions are managed when you post in a thread.
By default, the system is set to automatically subscribe you to threads you post in and notify you immediately by email.
You can change that option such that it notifies you less frequently in "digest" format (multiple notifications in the one email), or you can turn off automatic subscription altogether.
In the control panel, you can also see a list of the threads you have subscribed to and manually unsubscribe to them. Alternatively, when viewing a thread you are subscribed to, look in the "Thread Tools" menu at the top and click on "Unsubscribe from this thread"