API for customization and integration with other services

Some of my clients are long-time Outlook users and have custom-made AddIns installed that allow the integration of e-mail workflows with other local services (like querying a database to support archiving of mails in project folders). Since upcoming versions of Outlook are going to remove this feature (and don't get me started on storing my login data on their servers), I could make a good argument for switching to another email client provided such customizations are possible, too.