WP Mail plugin is simply a wp network mail or message system. User can send mail or messages to other users over one wp network.

All-In-One WP Mailer Plugin: background queue sending, mail open/click measurement, email address check, DNS setup, send emails 10x faster than SMTP

Server Mail Blacklist Checker checks if your website or server IP is listed in DNSBL (DNS-based Blackhole List) and RBL (Real-time Blackhole List) blacklists.