Hi a second time.
One problem I have solved by myself.
Sending emails to POP3s now works.
I had to edit /etc/postfix/master.cf.
The following lines need to be commented in (no "#" at start of lines):
tlsmgr fifo - - n 300 1 tlsmgr
smtps inet n - n - - smtpd -o smtpd_tls_wrap\
permode=yes -o smtpd_sasl_auth_enable=yes
587 inet n - n - - smtpd -o smtpd_enforce_\
tls=yes -o smtpd_sasl_auth_enable=yes
(lines 122 - 124 in my config)