First I installed sendmail as it used to work (with previous Suse
versions), for a permanent connection, providing an IP address
for the mail server. It didn't work; after many hours we found out
that the mail wouldn't go to the specified mail server.
So looking up POSTFIX_RELAYHOST in Yast/etc/sysconfic editor,
and there it is, it has the right IP address in it?! Then looking
it up in /etc/postfix/main.cf we see that the Yast settings have no
influence (at least regarding this variable) --- whatever we do with
Yast the value for relayhost in main.cf is the empty string.

Setting "relayhost = IP-address" directly in main.cf solved the problem.