Very odd, eMail sender listed as _, plus more..
 

So, something weird is happening, regardless of having my webmaster@example email in contact details, everytime someone is emailed from my forum the sender is listed as _. Also, when someone registers, at the top left hand corner of the navbar right above "Thank you, proceed to edit Profile, blah blah" it shows a white _. When I receive the New User emails, the referrer is listed as _, unless the user specified a username in that space.

Any ideas? This is quite annoying.

Is the "_" meant to denote a white-space, or an underscore itself?

An underscore itself.

Sounds like you have set your Site Name to "_".

Nope, site name is perfectly fine within the admincp & for verification, it show's at the top of the web browsers title correctly as well.

Any other ideas?

Did you try to disable the plugin system via vbulletin options and send out an email and see if the issue is gone?