New member verifications are not sent thru the mail table but most other mails are.
The table name is ow_base_mail what happens is that the mail is sent there first, then when the cron runs it removes it from that table and sends the mail. So the mail is not in that table for very long and its gone. Once that happens it is out of the Oxwall control and is up to your server and then your email host.
If you have a dedicated server or VPS you can check your WHM or whatever host manager software you use to check the outgoing mail from the server to see if your email has been sent from the server. If so then it is up to your email client after that.
First step verify that your cron is running
then check that table to see if there is anything in there
if that table is full of stuff then your cron is not running properly
then check the server