It is single-threaded, although it doesn't interact directly with SMTP, it passes a command to a different machine. The other machine actually forms the mail message.

The program is stopping printing right in the middle of printing a variable. This is the sort of thing that happens when you run out of disk space, although I have plenty of space in the filesystem.

The process can hang for days without moving. It took me a couple of days before I noticed the problem because there were no obvious signs of errors such as stderr, stdout, logfile entries.