Hello, I'm Alicia. Thanks for your question. I'm happy to help you today.
If there's no other reason to be suspicious, your husband is faithful and loyal to you, and you have an otherwise happy relationship, I wouldn't give this a second thought. Some people might think it's a bit unusual to start an email to an ex using those words, but if they have an amicable relationship and stay in contact from time to time, then it could simply be a friendly way of addressing each other. Exes usually have long histories with each other and this form of address could just be habit. I know that it's not ideal and it might make you feel insecure, but if that's the only cause for concern, I would dismiss it. If it makes you feel more comfortable, you could bring it up with your husband in a nonchalant way, but that depends on you and whether he knows if you saw the email (and why you saw the email, if it was a private email account - addressing it could make matters more sticky for you.)
But in cases like this, it's usually best to let it lie unless other issues are present that cause you to have doubt or suspicion, or if it's causing you a lot of anxiety
and grief. But from what you've said, it sounds harmless.
I hope that helps. Best wishes.