With claws-mail, there is a sylpheed fork available in Debian that has the porting to GTK 3 done. sylpheed upstream development seems to have stalled. Please consider to have a good migration story for users and get rid of sylpheed for trixie.
- Bug#967764: sylpheed: depends on deprecated GTK 2 Bastian Germann
- Bug#967764: sylpheed: depends on deprecated GTK 2 Alexandre Detiste
- Bug#967764: sylpheed: depends on deprecated GTK... Ricardo Mones