While claws-mail (CM) is composing message, he saves one periodically in drafts (AFAIU). I have a habit to save (Ctrl+S) a message when I compose it. CM obeys me and saves it, too. So, I finish to write my message I have a large amount of partially written messages in my Drafts folders. I would not complain if CM remove them, but it does not. I guess, there can be the only one draft message while user is composing a message, which (draft) should be removed after user send or defer sending of a message.
That's gmail, I suppose?
bingo! )) Yes, it is Is something wrong with GMail?
Yes. gmail can't handle incremental saves to drafts. There are 2 things that you can do: Add a local MH mailbox (/File/Add mailbox/MH...) and use its drafts folder instead. Turn off the option "Automatically save message to drafts folder every n characters'. (/Configuration/Preferences/Compose/Writing)