SP Mail Client
SP Mail Client is a webmail standards-based Joomla! extension, with your site's template-like user interface. It utilizes Joomla! access control, contacts and template for rendering. It works both in back end and front end, has very few requirements and is very easy to configure.
Each Joomla! authorized (e.g. registered) user can access all his/her IMAP email accounts, including GMail, Yahoo, and any other mail server that supports IMAP, from one web page. It support's Bootstrap responsiveness, so it does not matter if you access it from your PC, or smartphone. The rendering is handled by your site's template.
SP Mail Client has all the functionality you would want from an email client, including strong MIME support, contacts, and folder manipulation, plus all Joomla! features.