Thank you for your hints. I added the following to my stunne.conf:
;TLS Version (Strato since 7/2021)
sslVersion = TLSv1.2
The first trial failed, because in the General Settings of pmMail 3.24, page Secure Transfer, there was still an old stunnel.exe version selected.
Then I realized that if no sslVersion switch is selected, the new stunnel.exe version also works. It seems that it select TLS 1.2 as default.