In KDE Trojita 0.7, 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://bugs.kde.org/show_bug.cgi?id=404697