When sending a mail to many recipients the 'To:' header can become longer than the allowed 998 bytes. KMail should fold it to span several lines so the mails don't get rejected by mail servers.
STEPS TO REPRODUCE
1. Create a new mail.
2. Add many recipients.
3. Send it to someone using (for example) exim.
4. Look at the reject log on the mail server.
Mail gets rejected due to header not valid according to RFC 5322.
Mail gets delivered because it is not malformed.
Linux/KDE Plasma: (whatever Debian/sid ships)
(available in About System)
KDE Plasma Version: (whatever Debian/sid ships)
KDE Frameworks Version: 5.54.0
Qt Version: 5.11.3
 RFC 5322: https://tools.ietf.org/html/rfc5322#section-2.1.1