When KMail composes an HTML reply to a message, it also sends a plain-text version (as it should). In this version, quoting is broken. This matters mostly in two scenarios:
1) Mailing lists and their archives
2) A two-way exchange, when I prefer plaintext. Even if I "reply using HTML if present", I still see incoming messages first as plaintext. So if the plaintext in my reply to them has broken quoting, I will see broken quoting in their next reply to me.

Reproducible: Always

Steps to Reproduce:
0. In Composer, set "options->Formatting(HTML)" to on -- or check the configuration option "reply using HTML if present"
1. Reply using HTML, quoting the original message
2. Open your reply message, and look at its plaintext version

Actual Results:  
Missing any quotation mark, except for a header "This and that said". If replies are interweaved with original text, no way to tell which is which.

Expected Results:  
Quotation marks (">" or whatever the quote mark is) on quoted lines.

This may or may not be a duplicate of #321424 -- I cannot be sure what, exactly is reported there.
#316917 and #317227 are related.