MessageOne - MessageOne offers email management solutions such as EMS and AlertFind for archiving, e-discovery, security and notification services.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Vade Secure - Email security to protect against email-borne phishing, spear phishing, malware, and ransomware. Email security and management based on artificial intelligence.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
MailClark - The Slack bot for external communications
Mailing Group - Listserv - Allows you to run a free Listserv or email group right from your WordPress dashboard.