In Claws Mail 3.14.1, an attacker in possession of S/MIME or PGP encrypted
emails can wrap them as sub-parts within a crafted multipart email. The
encrypted part(s) can further be hidden using HTML/CSS or ASCII newline
characters. This modified multipart email can be re-sent by the attacker to the
intended receiver. If the receiver replies to this (benign looking) email, they
unknowingly leak the plaintext of the encrypted message part(s) back to the
attacker.

Reference:
https://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=4159