Step 1.

Go to your Steam profile page and press the "Edit profile" button.

Step 2.
Go to "My Privacy Settings" tab .

Step 3.
At the very bottom of the page there is a switcher called "Confirmation of Trades". Set its state to "Disabled" to disable the email confirmation and vice versa. 
Step 4.
Press  "Yes" button.

Step 5.
Save changes.

Step 6.
Check your email inbox.