When replying to an email address that is configured as an account in Kmail, the Reply button inserts the To email address into the To field, instead of the From address into the To field. This behavior is tricky, because when I replied to the first account, the second account was actually replying to itself! Very confusing. Reproducible: Always Steps to Reproduce: 1. Set up Kmail with two (or more?) email accounts. Let's say the accounts are aaa@mail.com and bbb@mail.com. Set up an identity for each email address. 2. Send an email from aaa@mail.com to bbb@mail.com (be sure to use the appropriate identity). 3. In the Inbox of bbb, select the received message and press the Reply button (or press the R shortcut key). Actual Results: I get bbb@mail.com in the To field. This means bbb@mail.com is sending a message (with his own identity) to himself. Expected Results: I expect that the To field would show aaa@mail.com, because that is the address in the From field. Note that you can do the reverse: send an email from bbb to aaa, and reply to bbb from aaa. You'll find that aaa@mail.com is in the To field instead of bbb@mail.com. It only seems to matter that both accounts are configured in Kmail. For email addresses not configured as accounts, the Reply action puts the proper email address in the To field. I see nothing in the header of my mails to that makes this behavior understandable. It's not a mailing list.
I have also found this bug in KMail 5.1.3. If I send myself emails from any one of my identities to any one of my other identities, and then try to reply to those emails, the default is to reply to the address that *received* the email that's being replied to, rather than the address that *sent* it. Deleting the identity that sent the email restores proper reply behaviour. Re-creating the identity restores improper reply behaviour. If the email contains a Reply-To header, then replies go to the Reply-To header and this bug is not triggered. This bug seems to have been reported previously in #301449 "Replying to a mail sent by another identity in the same kmail copy leads to replying to the current identity itself". That bug was not confirmed in a supported version of KMail, so it was RESOLVED UNMAINTAINED. I found it difficult to find this bug report, because the title is not very descriptive, so I am taking the liberty of changing the title to make it a bit more searchable.
Fixed in 5.13.0