As you see in this image I don't have the menu of KMail. Really I don't know how it disappeared. What must I do for getting it, again?

Thanks.

ctrl+m will toggle the menu bar

Does not work, I already had tried that out

In other applications that key combination works...

close Kmail and edit file $KDEHOME/share/config/kmailrc $KDEHOME/share/config/kmail2rc
- find the section [general] and make sure the line ShowMenuBar= is set to true

The line did not exist, I have added it and set to true (in general section), but still ctrl+m does not show the menu.

I checked again and that line only appears when the value is false (menubar set to not show)

can you try as a different user and if it works, if so then I'd suggest renaming your rc file with kmail closed and see if that fixes it

I have tried out with a different user. I see the menu bar KMail, but by pressing ctrl+m it does not dissapear. This key combination neither works there. Perhaps, is a bug of Linux Mint.

sorry, no idea if it's a Mint issue, you could ask on their forums

are you running an old version of KDE? upgrading shouldn't hurt and might help