How do I change the format/ display of the list of mailboxes/ folders (or any menu)?
The default display of mailbox folders includes a lot more information than sometimes needed. It is "N permissions owner group size date name" but you might prefer something shorter like "N size date name". You need to adjust the $folder_format variable to configure the look of the display of available inboxes (browser), look it up in the manual.txt. In general, you customize views with the *_format set of variables, there are some of them.
set folder_format="%2C %t %N %8s %d %f" # what I settled on
Can I personalize/ shortcut the display of or label the folder names in the folder browser?
Either try your luck with /RemoteFolder? hints for shortening pathnames via $folder, or name the folders as needed (for sorting or saving typing).
Why doesn't $index_format "%b" show some Maildir?
Because "%b" extracts the string after the last slash of the mailbox. If you want to see the string "mail" for "%b" on "/home/you/mail" Maildir, you have to go to "/home/you/mail", i.e. you should not go to "/home/you/mail/" (there is nothing after the last slash).
Mutt has also a similar bug. Mutt's directory browser does not allow you to go to ".." when $folder is "/home/you/mail/". This is bug#1885.
Conclusion: Be careful not to add "/" after Maildir (or MH) folders.
Can Mutt show the (new) message count in the folder browser?
It depends on the folder type:
- IMAP - yes, but not all servers support it
- mbox - no
- maildir - no
See "folder_format" for details, %N will give the number if supported. For maildir, you can try the "sidebar" patch from the PatchList. It isn't supported by the Mutt developers, though.
How to make the From-addr shown normally in index-view to show To-addr for stuff that I sent?
You have to setup the addresses by which mutt should identify you as sender by using "alternates" and see also "index_format" %-codes.
Can mutt use virtual folders/ mailboxes?
No, not directly.
The best you can get only using mutt is defining "mailboxes" for folders receiving new mail, and have those listed in the folder-browser (with indication of which one has new mail: see folder_format) by pressing "c?TAB" (by default, otherwise "c??" and search for "toggle-mailboxes").
External tools such as mairix or nmzmail can be used for this purpose. With them, you can create search patterns (you can view them as filters) that will create copies/symlinks of your messages into a mailbox of your choice. See UserStory/SearchingMail?
Why are some 'N'(ew) messages flagged 'O'(ld) when I leave the folder containing them and return?
When you read a msg, mutt removes the "N"ew flag from it. When you quit mutt, it can happen that you haven't read _all_ new msgs. mutt will mark those "seen-but-not-read" as "O"ld for the next run, to remind you that they arrived already in the previous session but you haven't read them yet. Old == New (unread) from last session. "mark_old" controls wether to distinguish "O" from "N" or not.