Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Roundcube - Web-based IMAP email client
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Skizzle Email - Skizzle is the most secure way to share end-to-end encrypted files through email, backed by blockchain