si.Mail is described as 'Published under GPL (you don’t have to pay for it if you are using it, full source code availability)' and is a Email Client in the Office & Productivity category. There are seven alternatives to si.Mail for a variety of platforms, including Windows, Linux, BSD, Mac and PortableApps.com. The best alternative is Thunderbird, which is both free and Open Source. Other great apps like si.Mail are Microsoft Office Outlook (Paid), Claws Mail (Free, Open Source), Kontact - KMail (Free, Open Source) and Sylpheed (Free, Open Source).
KMail is the email component of Kontact, the integrated personal information manager of KDE. Kontact is a personal information manager and groupware software suite for KDE. It supports calendars, contacts, notes, to-do lists, news, and email.