I have to agree, opening and reading the email is breaking your own boundary of no contact but as codies I do understand your deep desire to “know” what’s going on with him.
Was there any substance in this email regarding you? Was there any apologies for his behaviors prior to rehab? Any mynute morsal of remorse or was it all about him like usually?
Much like Kryptonite that can injure or kill superman……….guilt is a codies kryptonite.
Try working on getting control over your guilt… when guilt is running our lives we’re in big trouble!