Add the newsletter email address to your contact list.

If one of my mails has been moved to the spam folder, you can mark it "Not Spam" to whitelist it.

Select contacts from the options on the left side of the Gmail Inbox. Enter the email address in the primary email box.

Without an example email address in your inbox:
Select Actions from the toolbar at the top of the screen.
Select Junk E-mail.
Select Junk E-mail Options...
Click the Safe Sender tab.
Click Add.
Type in the email address you wish to add to your safe sender list.
Click OK.