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Like Twitter, the <a href="https://wordpress.org/extend/plugins/buddypress-followers/" rel="ugc">BuddyPress Followers</a> plugin adds the ability to “follow” users on your BuddyPress site.
However, by default, any member on your BuddyPress site is allowed to send private messages to anyone in your userbase.
This plugin allows members to send private messages only if the recipient is following them. This will help prevent private messaging spam on your BuddyPress site.
<h3>Recommendations</h3>
I do not recommend using the BuddyPress Followers plugin in tandem with the BuddyPress Friends component.
If you’re looking for a similar plugin for the BuddyPress friends component only, check out <a href="https://wordpress.org/extend/plugins/buddypress-private-message-for-friends-only/" rel="ugc">this plugin</a> instead.
<h3>Donate!</h3>
I’m a forum moderator on the buddypress.org forums. I spend a lot of my free time helping people – pro bono!
If you downloaded this plugin and like it, please:
<ul>
<li><a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=S6EZYK894S99G" rel="nofollow ugc">Fund my work soundtrack!</a> Music helps me help you! A dollar lets me buy a new tune off Amie Street or emusic.com! Or if you’re feeling generous, you can help me buy a whole CD! If you choose to donate, let me know what songs or which CD you want me to listen to! 🙂</li>
<li>Rate this plugin</li>
<li>Spread the gospel of BuddyPress</li>
</ul>

BuddyPress Private Messages for Followers Only

Allow members to send private messages only if the recipient is following them.  Requires the BuddyPress Followers plugin.

Jonathan Lau

CubePM is a private messaging system for WordPress.
Unlike other PM system, CubePM allows messages to be sent to multiple users and also displays the whole PM system on the frontend using a short-code.

CubePM

CubePM is a private messaging system that integrates right into the frontend of your site.

BuddyPress Private Messages for Followers Only vs CubePM