Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
sSMTP - sSMTP is a simple MTA to deliver mail from a computer to a mail server.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.