You’re coworker probably has the same IP address. So either way it traces back to your work….and I’m sure that you don’t want someone calling your work asking who sent them an e-mail.
Of course, there are many other “clues” that people don’t take into account when they try to send “anonymous” e-mails. Things such as grammar, verbs they commonly use, lack of capitilization & incorrect usage/lack of punctuation can all give clues as to who sent the message. Assuming he has received (or will receive) other e-mails from your actual account.