Ok. The output of your command shows:
mail:~# openssl s_client -starttls smtp -showcerts -connect mail.leagues-united.de:587
CONNECTED(00000003)
depth=2 C = US, O = Internet Security Research Group, CN = ISRG Root X1
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = R11
verify return:1...