Quick poll :)
I discovered Sylpheed then Sylpheed-Claws in December 2001. Started with Sylpheed version 0.6.6 (GTK+ 1.2.10; ), then after one month and two patches integrated in -Claws, switched to Sylpheed version 0.7.0claws (GTK+ 1.2.10; i686-pc-linux-gnu).
Didn’t look back since then…
What about you? (use an extended search in your sent folder, like:
header « X-Mailer » matchcase « sylpheed »