Writing emails in a full window

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C-x 1 will close hide all the other windows and just give you the current highlighted buffer. If you want to go back to the previous window layout you could then use winner-mode.

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Via frame parameters, I think. Not sure about specifics for rmail.

You might find this helpful: https://www.masteringemacs.org/article/demystifying-emacs-window-manager Particularly the bit on display-buffer-alist.

RMAIL-MAIL-NEW-FRAME is a variable, not sure how you are invoking it? `emacs -Q` will open a half window if you hit `m` in a RMAIL buffer, indeed. You might want to check COMPOSE-MAIL and friends?