The first item to test would be that your email account in ubb is working.
You could test it by say using the email post button when viewing a post. And see if you receive it.
Second would be checking
Control Panel » Primary Settings
Board Email Address:
All emails being sent out will come from this address.
This needs to be a valid email address
Third
Select the advanced option below the above reference and double check the
Use specified SMTP Server:
Normally this is controlled by php.ini. However if you need to send mail through another SMTP server and cannot do this by editing php.ini, you can specify it here.
Forth
Could be your board email address is blacklisted by the member ISP
