Hello I have difficulties of solving this problem, after a mebbers registration, I get the 500 error internal server, when I check the real error by turning the dug mode, I get this error message...
When SMTP is enabled, ALL emails are sent by Cron.
So make sure that your Cron is configured properly.
Cron set up documentation can be found here: http://docs.oxwall.org/install:cron
Not mandatory, but I also suggest checking this topic to get an idea on how Cron and mailing are connected in Oxwall: http://www.oxwall.org/forum/topic/10502