Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems. Exchange Server primarily uses a proprietary protocol called MAPI to talk to email clients, but subsequently added support for POP3, IMAP, and EAS. The standard SMTP protocol is used to communicate to other Internet mail servers. Exchange Server is licensed both as on-premises software and software as a service (SaaS). In the on-premises form, customers purchase client access licenses (CALs); as SaaS, Microsoft charges a monthly service fee instead.