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.

Shamim Hasan

Front End PM extension to integrate with Ultimate Member. This works out of the box, no settings required.
<a href="https://wordpress.org/plugins/front-end-pm/" rel="ugc">Front End PM</a> 
<a href="https://wordpress.org/plugins/ultimate-member/" rel="ugc">Ultimate Member – User Profile & Membership Plugin</a>
This two plugins need to be activated first before activating this extension.
Once activated, you will see “Message” Tab in user profile and account page. If it is own profile then clicking “Message” will show full message system, if it is another user profile then clicking “Message” will show a form which will send message directly to that user.
<a href="https://www.shamimsplugins.com/docs/front-end-pm/integration/front-end-pm-ultimate-member-integration/?utm_campaign=wordpress&utm_source=readme&utm_medium=description" rel="nofollow ugc">More Details</a>
Github
<a href="https://github.com/shamim2883/front-end-pm-ultimate-member-integration" rel="nofollow ugc">https://github.com/shamim2883/front-end-pm-ultimate-member-integration</a>

Front End PM – Ultimate Member Integration

Very much useful extension

Front End PM extension to integrate with Ultimate Member

CubePM vs Front End PM – Ultimate Member Integration