Ok, thanks for these hints.
Telnet seems to work:
[root@alfresco1 ~]# telnet 10.2.1.195 25
Trying 10.2.1.195...
Connected to 10.2.1.195.
Escape character is '^]'.
220 mail1.blabla.blah ESMTP Postfix
and in the maillog the following lines are written:
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: > unknown[10.2.1.198]: 220 mail1.datacave.ch ESMTP Postfix
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_create: SASL service=smtp, realm=(null)
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: name_mask: noanonymous
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: Connecting
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: VERSION?1?1
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: MECH?PLAIN?plaintext
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: name_mask: plaintext
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: MECH?LOGIN?plaintext
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: name_mask: plaintext
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: SPID?36733
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: CUID?4
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: COOKIE?379b061dbad47940d769862db9b0c809
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_connect: auth reply: DONE
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_mech_filter: keep mechanism: PLAIN
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: xsasl_dovecot_server_mech_filter: keep mechanism: LOGIN
Jun 5 20:53:19 mail1 postfix/smtpd[36866]: watchdog_pat: 0x7fc02f6c4180
Jun 5 20:53:53 mail1 postfix/smtpd[36801]: disconnect from unknown[10.2.1.194]
The disconnect comes directly after entering 'quit' in the telnet session. I do not really understand what is going on there, I am not a postfix expert, but maybe somebody else can read and understand it. I still cannot send a mail from any server other the postfix server itself.