Let's not try to complicate the matter. Email is just text and there is not a lot you can do with it. Let's leave the guidelines as is, and get on with more important issues. If a blind person wants to turn off the reading of the greater character, that is up to him or her. The most helpful item is to put your initials before what you are saying.
People can search for that and get through the message in about the same time as a sighted person.