Message Lib instead of Mail only ?

Both can exist ... if there's a generic messaging library, it could have an email adapter built on top of Email\Message.

Its an option yes.
 
Those days almost all sites built have integration of some  3rd party application with messaging/notifications or SMS verification and etc. (all the time sending some messages to somebody out of the site)..

I think that replacing the email with messenger with email "adapter" and providing the interfaces to build other type of messenger adapters is a good idea for this new major CI version . Emails performance won't be worse if it follows such structure, just instead of loading an email lib it will be a messenger factory > email.

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