I broke up with my ex about a year and a half years ago. We tried staying friends for nearly a year, but it mostly resulted in us hooking up / having arguments. The arguments were a result of me finding out he lied about certain things during our relationship - nothing too serious but it made me go crazy knowing he'd been dishonest with me. Around 6 months ago we had finally managed to just be friends and we were both really happy about this. He was supposed to come visit me (I live a couple hours away) but it never happened due to me being so busy. Anyway, 5 months passed and we didn't communicate at all during this time. I'd started to regret the way I'd ended the relationship and I finally could understand how he felt during the breakup. I found out he had a new girlfriend and I felt genuinely happy for him. I sent him an email asking him how he'd been and let him know what I'd been up to. He actually emailed me back which really surprised me.
He sent the email a month ago and I never responded. I just wasn't sure if that would be appropriate. I really miss him a lot at times. He was the best friend I ever had but I'm not sure if it would be right to keep trying to talk to him since he has a new girlfriend. I don't want to get back together, but I do miss him and if he'd like to be friends again that would be great.
What do you think? Should I respond to his email or just leave it alone?