Option to use stopforumspam

[sebix]

We operate a public wiki which suffered from a wave of spammers until we had to restrict the registration. We analyzed the email addresses and found that many of them are in the database of stopforumspam.com.

Using the online API, 81% of them were identified as spammers.

Using the (Toxic Domains Database](https://www.stopforumspam.com/downloads) the rate was at least 58% (The API performs more checks than simple string comparisons).

What do you think about an option to check the registrant's email address domain against the database (API or local database copy)? The same function could also be used for a local email address domain blocklist or whitelist.

[RogerHaase]

agree, add as an option