Sylpheed is described as 'simple, lightweight but featureful, and easy-to-use e-mail client (mailer, MUA). Sylpheed runs on many systems such as Windows, Linux, BSD, Mac OS X and other Unix-like systems' and is a popular Email Client in the Office & Productivity category. There are more than 25 alternatives to Sylpheed for a variety of platforms, including Windows, Linux, Mac, BSD and Android. The best alternative is Thunderbird, which is both free and Open Source. Other great apps like Sylpheed are Microsoft Office Outlook (Paid), Evolution (Free, Open Source), Mailbird (Paid) and Geary (Free, Open Source).
Historically, Icedove was a re-branding of Thunderbird by the Debian project, with minimal modifications, in order to resolve branding disputes. Debian since discontinued Iceweasel in February of 2017 after reaching an agreement with Mozilla over the use of it's trademark.