There is a way for users of your site and blog to contact each other is via email contact form, Now With WordPress Powered Blog and Website it is possible to provide your visitors with a way to send and receive private messages though these WordPress Plugins.
1. WP Private Messages
WP Private Messages is a plugin for messaging each others. It requires minimum user level 0. You can send or recieve messages, and of course, reply.
2. Private Messages For WordPress
This plugin allows users of WordPress blog send private messages (PM) to each other. The number of PM can be controlled in plugin option page.
Otherwise, an email will be sent to user when a new PM is received. Email template is full-controlled.
3. Cartpauj PM
Cartpauj PM (Private Messages) allows you to easily add a Private Messaging system to your WordPress blog/site. Unlike other Private Messaging plugins available, Cartpauj PM works through a Page rather than the WP Dashboard — This is very helpful if you want to keep users out of the Dashboard area! Please see the features and TODO lists below for more about what Cartpauj PM currently does and what’s planned for the future.
4. BuddyPress Private Messages for Friends Only
By default, any member on your BuddyPress site is allowed to send private messages to anyone in your userbase.
This plugin only allows friends and site administrators to send private messages. Thus, acting as another deterrent from illegitimate users.