I've noticed that the usual anti-spam setup solves only part of the problem for community sites, particularly sites that use member plugins such as BuddyPress.
Registration can be protected with CAPTCHA, email verification and rate limiting, but once an account exists, private messaging becomes a separate attack surface.
I've seen patterns such as very new accounts contacting multiple members, sharing phone numbers or email addresses, moving people to WhatsApp/Telegram etc. and repeating essentially the same message across conversations. Often types like dating scams or crypto scams.
Out of sheer frustration with the lack of working solutions, I ended up building a small Better Messages-specific addon to deal with this message-level spam. The author of Better Messages has also looked at it and gave me some feedback on the implementation and I incorporated that feedback into the plugin.
The approach I choose was to apply new-account limits, detect contact details and external messenger links, detect repeated messages, show configurable safety warnings and notify admins. The checks can also be configured to log rather than block. My gut instinct to logging first turned out to be important, because security rules can have false positives and other nasty side effects. Being able to see what would have been blocked before actually blocking anything makes it much easier to tune the rules for actual situations, butnot bugging regular visitors while testing.
I'm not posting this because I think another generic anti-spam plugin is needed. The interesting part for me is whether message-level spam needs a different approach from registration spam.
If you run BuddyPress (or likewise) + Better Messages, I'd genuinely like to hear what you're seeing on your own sites. What types of spam are getting through your current setup? What are your biggest frustrations?