Honestly I have been amazed with this email client. I tried it when it was in one of its early versions and I didn't like it too much back then. But now things change and for the better.
For starters it has everything I need and use in Thunderbird, and even more:
- Autocomplete email addresses.
- Filtering emails.
- It is minimized on the tray.
- Simple interface.
- Low consumption.
- Support for IMAP / POP / SSL / STARTSL.
- And it allows me to import emails MBox so I already have my messages from Thunderbird en Sylpheed.
- Among other things.
Installing it in Debian is as simple as opening a terminal and putting:
$ sudo aptitude install sylpheed sylpheed-i18n sylpheed-plugins