I want just to report that me and some other users (there are some topics in wordpress support forums) experienced some problems with registration email that were not sending.
We used SMTP plugin (no matter wich, i tried 4-5 of them) and Simple:Press.
The problem was registration mail were not sent while if u use “forget password” link they worked. To fix, I just disabled simple press email system.
If you are using a decent smtp plugin that sends out your emails as from your website instead of the default from ‘WordPress’ – then it is probably a good idea to deactivate the SP email overrides. However – you probably only actually need to turn off the use of the SP New User registration email which has been a problem for a small handful of people as it may attempt to access a data item that no longer exists. This is following the recoding by the WordPress team of their password handling.