Originally reported in: https://wald.intevation.org/forum/message.php?msg_id=5773
Looking at S/MIME mails in sent folder will lead to a "No Data" error and Thunderbirds builtin S/MIME support can't decrypt it. When the mail is sent to yourself it will also look broken and is not properly recognized as S/MIME mail
KMail decrypts the mail just fine.