This program is inspired in Du Yang's notifier program. But instead of polling on msmtp's log file and compare sizes to see if there any changes, it uses the kernel's inotify feature. Also, the notifications are permanent: the user is required to manually close them. This is to avoid having them disappear before he could read them.
Requires a kernel that supports inotify and, libnotify, and pkg-config. Clone to a directory of your choice, run make, and then run program. Default log file name is "./msmtp.log". To change, use the -l flag.