When you're configuring 'Mail Server' in YaST you're configuring postfix, so you may find looking at postfix documentation helpful.

Does the setting
relayhost = the.smtp.server.you.want.to.use
appear in /etc/postfix/main.cf ?

I don't have postfix configured to use an external SMTP relay but I did try setting it up a couple of years ago out of curiosity. Like you I noticed that an email containing cronjob output or sent from command line with the mail command would get delivered to an email address that ends in the same domain as my machine's hostname, but send to an external address such as @gmail.com and it sits in the mailq along with a message saying 'host unreachable' I just looked it again quickly and found that if I specify my employer's SMTP server in the YaST configuration the setting does not get added to /etc/postfix/main.cf. If I manually add the relayhost setting to /etc/postfix/main.cf then restart postfix (service postfix restart), then do

$ date | /usr/bin/mail -s "test $(date)" -r me@mywork.net me@gmail.com
that works. The SMTP server I'm using does not require authentication so I've only looked at the relayhost setting.