I think that could be very useful to be able to set different signatures for your emails depending on the recipient. I can't find how to configure it, I've just found how to configure one signature at Account Preferences -> Account -> Compose. Even it could be interesting can choose manually between different signatures. Is there any plugin that provides this functionality?
I'm actually using templates to manually add a signature depending on who I'm writing too (and including it manually every time). However I think that claws-mail should do it automatically, it's the biggest lack for me.
FWIW A (partial) workaround is to create an account for each signature, then select the appropriate account manually. If you sort incoming messages in folders, note that you can also specify a default account for messages composed when in a specific folder. claws-mail also automatically chooses the right account when replying to an email, but AFAIK this is based on the email address in the To:, Cc: or Bcc: field, so this won't work for several accounts with the same email address.