It is perfectly okay that you don't say anything to her about her drug problem. In fact, you would probably be saving yourself some pain and anguish. If she wanted to quit, she'd do something about it. Since she isn't doing anything about it, she doesn't want to quit. If you were to bring it up, most likely, she would simply become angry and "blame the messenger" with all the meanness and cruelty that an active addict in denial can muster. And her behavior would still not change. From a logical standpoint, the risk-reward just isn't worth it. You can't save her (or anyone else) from herself.
Is it okay to respond to emails and intermittent calls, even if it feels disingenuous? I might ask what about it feels disingenuous. Is it that you don't really want to talk to her on a social level? If so, then on what level DO you want to talk to her? Because, like her addiction, if you really wanted to change your own behavior, you could do so. I'm guessing that on some level you get something out of these limited exchanges. You limit them to avoid the destruction that more regular contact would do, which is a very good thing. And you must be getting something from the limited contact because you've proven you can limit contact, which means you could go to no-contact if that's what you truly wanted. Perhaps finding the answer to "what do I get from this?" will help you find peace.
I have limited contact with my parents. I do so because it helps me monitor what kind of shape they're in (in case I need to call social services in) and because it is a reminder to me that they weren't always this way. I limit face-to-face meetings and phone calls. I wish things were different, but they aren't. While it is not my responsibility, technically, to take care of them, I do feel a moral responsibility to check in on them every so often - just as I used to check in on the old lady who lived across the street from us, simply because she lived alone, was very old, and I had her son's phone number. Not that she was my responsibility, but that I am a human and I try to help my fellow humans who find themselves in situations that make them vulnerable.
In other words, I stay in contact with my parents because it makes me feel good about myself. Why do you stay in contact with your mom?