Address the reader using their salutation. Or address the letter "To whom it may concern" if you're writing a generic recommendation letter your co-worker could use when applying to any position.

In the past, obtaining recommendation letters was a requirement of the job search process. Today, not as much. Now, this step is considered optional, but savvy job seekers understand that it can ...

Dear Stuck: When you write someone a letter of recommendation, you are putting your own reputation on the line, at least a little bit. If you don’t feel good about writing one for this co-worker ...