The responses so far look very helpful. Please don't hesitate to ask
here for clarification or more information.
Like Ken, I use just one inbox. To select the value of the From: header
field for replies, I uses this in my replcomps:
%<(nonnull(getmyaddr{to}))From: %(myname) <%(getmyaddr{to})>\
%?(nonnull(getmyaddr{cc}))From: %(myname) <%(getmyaddr{cc})>\
%?(nonnull(getmyaddr{from}))From: %(myname) <%(getmyaddr{from})>\
%|From: %>
That looks for one of my addresses in To:, Cc:, and From:, and uses
the first that it finds. If it doesn't find me in any of those, it
puts an empty From: in the draft. I fill that in using editor
shortcuts.
Another nmh feature that might be of interest is its OAuth support
for sending and receiving through Gmail. See the send(1) and mhlogin(1)
man pages. And inc(1) can use it, as well.
David