Check your mail error log:
tail /var/log/mail.err
If you see:
sendmail[6059]: My unqualified host name (example-desktop) unknown; sleeping for retry
sm-msp-queue[6053]: unable to qualify my own domain name (example-desktop) -- using short name
The fix is to add .localdomain (or some fake domain) to your /etc/hosts
Before:
127.0.0.1 localhost
127.0.1.1 example-desktop
After:
127.0.0.1 localhost localhost.localdomain
127.0.1.1 example-desktop example-desktop.localdomain