Be sure to thank the person who made the connection and sent the original email as well. You can mention it in a sentence at the start of your reply.
Check the subject line of the email. If it’s straightforward and easy to understand, you can leave it. Otherwise, change it to something like, “Introduction to Jim Davis” or “Maria Vang recommended I get in touch. ”
If you get an introduction email during the weekend, it’s okay to wait until Monday to respond. You should try to use this opportunity to establish guidelines for communication. While you shouldn’t let that email sit in your inbox for weeks, you’re not going to lose the opportunity if you don’t reply to it in a couple of hours.