I've been looking over the online list, and have noticed the occasional guest sending e-mail to other members (it doesn't say which member), so I suspect that this may be a source of spammy e-mails.
I've looked through the options, but there isn't an option to prevent guests from sending e-mail. So, if your e-mail is public, I urge you to make it private.
Right now, aside from looking for a mod that would essentially hide public e-mail addresses, the only other solution I can think of would be to go through the memberlist and make sure that all e-mails are hidden. A mod would essentially do the same thing, only as a task that can be run.
So, if you're getting spam, HIDE YOUR E-MAIL.
This has been an RPM PSA.