I have had Motion servers send emails with jpg attachments in the past. I realized those systems were Debian with Motion running as root. In this Ubuntu system I'm running Motion from the stock repository, which runs as unprivileged user motion (which is a good thing). As I did with root back then, I'm using Mutt with command line arguments to send the message; this is the only way I know to email with an attachment.
The problem is that the motion user cannot send the email message. In testing with "sudo -u motion mutt" the program comes up and I create the message (with no attachment for test), but the program hangs when it tries to send the message. Ctrl-C breaks me out and that is as far as I get. No error message, and nothing in any of the log files to point the way. I don't see any particular place where permission to send email is being granted or denied in Postfix (assumed group permission, but it doesn't look that way). Mutt works as expected when run as a regular privileged (shell) user.
I could try to run motion as a privileged user by editing "NAME=" in the /etc/init.d/motion file, but I'd rather stay with the motion user if possible.
Not a problem with Motion itself, but I imagine others have encountered this or similar problems using Motion to email pictures or videos.