It is possible to mailbomb a third party by repeatedly posting the subscribe form.

[Mark Sapiro]

This can be prevented by refusing to pend a subscription when one is already pending, but that means if a subscriber loses or doesn't receive the confirmation request email, she has to wait PENDING_REQUEST_LIFE (default 3 days) before she can request another.

It can also be avoided by setting the list's subscribe_policy to Moderate, but that may not be desirable in many cases.

Because of these considerations, I will implement the refusal to pend a subscription when one is already pending, but make that depend on a new REFUSE_SECOND_PENDING mm_cfg.py setting.