Posteo - Web- and IMAP-based email service. 2GB cost 1€/month.
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
Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.
Teknik Mail - Webmail service. Powered by RainLoop.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.