Product information:
Kmail Version: 5.7.3
KDE Plasma Version: 5.12.3
KDE Frameworks Version: 5.44.0
Qt Version: 5.10.1
OS: Arch Linux x86_64

Bug Description:
As stated in 301449, when having multiple identities configured in Kmail and sending messages between them, Kmail will incorrectly set the recipient identities' address as the "To:" address as opposed to the sender identities' address when replying. This has been tested on multiple systems and clean installs and can be replicated each time.

Steps to replicate:
- Create two seperate identities in Kmail.
- Create two mail accounts in Kmail.
- Manually assign a different Outgoing Account to each identity under "Identity", "Modify" and "Advanced" (eg: assign foo@mail.com to the identity "Foo", assign bar@mail.com to the identity "Bar").
- Manually assign a different Identity to each account under "Accounts", "Modify" and "Advanced" (eg: assign "Foo" to foo@mail.com and "Bar" to bar@mail.com).
- Send a test email from "Foo" to the address in "Bar".
- Upon receiving the email in "Bar", hit "Reply".

Expected Results:
- "To:" field would be auto-filled with "foo@mail.com".

Actual Results:
- "To:" field is auto-filled with "bar@mail.com".

Workaround:
Upon replying, select "Reply to Author" as opposed to "Reply". This will assign the correct "To:" address.