I work in development and support for one of my company's products. I'm the only support employee, but there is another person that is above me. He's not technically a support employee, but he's tasked with giving me assistance since he has a lot of inside knowledge.

Apparently he worked on a small project, related to the project I support, for a customer, but it got stored away. Another customer wants a similar project and so this employee was supposed to dig it out and update it.

Several weeks ago the customer emailed my co-worker a couple times about the project and I was CC'd. In the email he directly addressed my co-worker, as in "Good Morning Jake,". Today I was pulled into my boss's office and scolded for not replying to this customer's email. Apparently, my co-worker never responded to him. I vaguely remembered the email, so I had to check it after the conversation with my boss. The reason I didn't reply was because it was directly addressed to my co-worker and I knew nothing about this side project that he was working on, even though it was slightly related to the product I support.

Should I go back to my boss and try to defend myself since I know more about the exact circumstances surrounding the email? I believe he favors this employee since he's been with the company since it was founded and I've only been here for 3 years.