SUMMARY
Sending mail with the default identity even if we choose another one. 
Only the identity used as default can send email properly.
The IMAP and SMTP settings are correct. I am not using crypto or signature.

(sorry if I may not have choosed the correct component for reporting the bugs)

STEPS TO REPRODUCE
1. Create Identity A for Email A (with IMAP & SMTP settings). Set this one by as the default address. No crypto set.
2. Create Identity B for Email B (with IMAP & SMTP settings). 
3. Send an email, the windows will open with the Identity A as sender (normal it is the default one). Change identity because you wish to send email from the Identity B, write a radom message and send.

OBSERVED RESULT
When sending the email, the sending is denied as it tries to send the mail with the default Identity A while we clearly choose to send it from the sender Identity B (that is not the default one).
If I set the Identity B as default, it works, but the problem is reversed if I want to send with Identity A... 

EXPECTED RESULT
If you send email with Identity A, it sends from sender Identity A. It uses the SMTP settings for the email of Identity A.
If you send email with Identity B, it sends from sender Identity B. It uses the SMTP settings for the email of Identity A.

SOFTWARE/OS VERSIONS
Windows: /
macOS: /
Linux/KDE Plasma: Manjaro  
(available in About System)
KDE Plasma Version: ??
KDE Frameworks Version: Version 5.112.0
Qt Version: Version 5.15.11 (built against 5.15.11)

ADDITIONAL INFORMATION
The xcb windowing system
KMail Version 5.24.3 (23.08.3)