E-mail Validations not Arriving

Some (many) users trying to sign up to my site aren't getting their validation e-mails.  I tried with a Yahoo account and never got anything in the inbox or junk folder.

I tried resending to no avail.

How can I figure this problem out? Is there a way to send a test e-mail to one of the accounts that worked?

Is there logging information for the plug-in?