MessageOne - MessageOne offers email management solutions such as EMS and AlertFind for archiving, e-discovery, security and notification services.
WP Mailster - WP Mailster enables you to run a listserv-like email discussion list on your WordPress site.
Vade Secure - Email security to protect against email-borne phishing, spear phishing, malware, and ransomware. Email security and management based on artificial intelligence.
Listeron - An email address for your group. Discuss. Debate. Organise. All with email.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Sendy - Sendy is a self hosted newsletter app that sends emails 100x cheaper via Amazon SES