I can’t tell you how many truly awful letters I’ve had to read in my life. As a former administrative assistant, I was responsible for all the unsolicited submissions to a major New York museum. Artists and collectors all over the country (and abroad) wrote to have their work considered for display or acquisition, and to be honest the decision frequently rested more on the quality of their cover letter than on their work _ which, romantic dreams aside, rarely if ever sells itself.
You can also include links to online portfolios, if appropriate, or a link to your professional social media account such as LinkedIn or Twitter. You don't want to make this section too cluttered, however, so restrict yourself to the most relevant information. Here's how to set up an email signature, along with more advice on what to include in it and what to leave off.