The key combination assigned to close the message compose window (Shift-H) is a problem because it prevents one from starting a sentence with a capital H.
This is not the default combination; you probably assigned it yourself. See: http://www.claws-mail.org/faq/index.php/Interface#How_can_I_change_the_key-bindings_.28hot-keys.29_in_Claws_Mail.3F
*** Bug 1316 has been marked as a duplicate of this bug. ***
*** Bug 2680 has been marked as a duplicate of this bug. ***