Mutt/Neomutt with Maildir do not save sent email

Solved, I moved ~/Maildir to ~/old_Maildir reinstalled neomutt and changed "sent" and "drafts" line in ~/.neomuttrc with:

set record="~/Maildir/sent"
set postponed="~/Maildir/drafts"

The + or = at the start of folder names will be replaced by the directory pathname that you have assigned to folder.

Since you set folder to ~/Maildir and later set record and postponed to +.Sent and +.Drafts, respectively, the sent mail will be stored in the Maildir folder .Sent under ~/Maildir, and any postponed messages will be saved to .Drafts in the same directory.

Note that .Sent and .Drafts are hidden names and won't appear in the default listing of ls unless you use ls -a or ls -A.

Example: Start mutt and configure it to store sent mail in .Sent under the main folder ~/test:

mutt -e 'set folder=~/test; set record=+.Sent'

I then send an email, and see what happens:

$ tree -a ~/test
/home/myself/test
└── .Sent
    ├── cur
    │   └── 1739548720.32991_1.myplace:2,S
    ├── new
    └── tmp

5 directories, 1 file

Conclusion: If you look for the two directories ~/Maildir/.Sent and ~/Maildir/.Drafts (in your original setup), you will notice that they exist (assuming Mutt had a reason to save anything to them).

As for your other issue, opening a message file from a Maildir folder is not something you should expect to do in Mutt. If you use the c command in Mutt to change folders, it expects you to pick a Maildir folder, not an individual message file from within such a folder. Changing to the folder ~/Maildir (or to just + or =, since this is the value of folder) with c would have shown you the new message in the index view.

Note, too, that the value you set for mask appears to be the same as the default value for that variable, and it makes Mutt avoid showing hidden names like .Sent and .Drafts whenever browsing the directory structure.