We built our linked mailbox functionality to sync with Gmail, and we use Nylas Mail, a third party provider, to support other email service providers such as Microsoft Exchange, Office 365, Outlook, Yahoo!, and custom email accounts/servers.
Keep in mind that Nylas does not support connecting to accounts via POP / POP3. The vast majority of mail providers that support POP3 also support the newer, better IMAP standard (SMTP is also supported).
Given that Nylas does not yet sync with large transactional sending services like Mandrill or Amazon SES, we won't be able to support those services through Nylas. However, if you'd like to send larger-volume emails than our recommended safe Sending Limits, you'll want to integrate your linked mailbox (whether Gmail, Microsoft Exchange, Office 365, Outlook, Yahoo!, or a custom email account) with Sendgrid and follow our instructions here to set it up.