Perforce mail address resolver is always returning a string, no matter what the actual email address is. I don't know what is the order how Jenkins loops through the various MailAddressResolver instances but if some instance returns an actual string (i.e. non-null), it will stick with that. I don't think it's feasible to really check whether the email address provided by Perforce is valid, but PerforceMailResolver could check for some value (e.g. "n/a") and return null if it matches. At least with Perforce 2011.1 you can't have an empty value for an email address.
There could be an UI element for configuring which email address would be thought as invalid, but I'm fine with hardcoding "n/a" there and documenting it somewhere. Would this be ok?