I think in the "not only / but also" structure, you can leave out "but", or even "but + also" altogether, but not just "also".
I have not only sent some emails, but I have also left some voice messages.
I have not only sent some emails, I have also left some voice messages! <-- Seems kind of emphatic to me
I have not only sent some emails, I have left some voice messages! <-- Seems kind of emphatic to me
I have not only sent some emails, but I have left some voice messages! <-- I'm not sure about this one. Using "but" alone, is it still idiomatic?
Just my opinion.