Well, assuming you've read all of the above, here are two final notes on
mail sending with
nndiaryis a real mail back end. You really send real diary messages for real. This means for instance that you can give appointments to anybody (provided they use Gnus and
nndiary) by sending the diary message to them as well.
nndiaryalso has a
request-postmethod, you can also use C-u a instead of C-u m on a diary group and the message won't actually be sent; just stored locally in the group. This comes in very handy for private appointments.