I think that membership should not require explicit approval, but pre-moderation of the first few posts would be reasonable. Inability to post-moderate is a problem, but that needs new software.
I believe all new members *are* moderated until one of the moderators explicitly decides they can be trusted (which usually only takes one or two good messages, I think). If by "post-moderate" you mean removing bad emails without requiring all emails to be pre-approved, that's impossible, with any software, since it would involve deleting emails off mail servers around the world...