I have installed OpenSuse Leap 15.6 with KDE3 Vers.3.5.10 and cannot establish a connection to the T-Online server via KMail. It works without any problems under OpenSuse Leap 15.5. I have the OpenSuse Repro: KDE3, 15.6
This version is not maintained anymore, report issue to the KDE:KDE3 maintainers