Message Lib instead of Mail only ?

You don't quite get what I'm saying - replacing is pointless. It will solve your problem, but will make others unhappy.
If I only want email, I don't want needless abstractions forced upon me.

CI\Email must remain stand-alone. The point of adapters is to act as mediators, not concrete implementations.

I understood you right the first time. 

My suggestion is based on the current trends at the web apps. 
In the past "Send Message to user" meant in almost 100% of the cases - send Email. But today its not the same. 
Its most likely that there will be 1 small "needless abstractions" but I don't think that it will be so big deal to blacklist the idea.
Sending an email is usually not so fast process, and some abstraction won't deal much delay..

The benefit will be big for those who are sending messages not only as emails (and this group is not small and its growing) .
It will be 1-2 lines of code for sending messages to any kind of 3rd party app (Gmail as mail, FB as notification,some GSM as SMS) 

I think that we should think out of the box, not only about the most regular and simplest case.