The contact us feature has stopped working since upgrading to 1.7.2. Our SMTP settings are correct and pass the built in test function.
This plugin was working prior to upgrading
Please check whether your cron jobs are properly configured as well as SMTP
Where do you get the error 'mail not delivered' ?
before that open your database in phpmyadmin
ow_base_mail table see if the mail goes away from that table. Seems like your cron is not working.
You can also check ow_cron/cron_error_log file if there's one, probably something is there too.