OK, I got a step further. I saw that i used the external machine names (server.somedomain.com) instead of the internal names (server.somedomain.local). Also on one server selinux was on. I changed it to permissive mode. These boxes work now. There is still one box causing a problem:
On the client it return the very meaningful error message:
'There was an unexpected problem with sending the email: The sender parameter must either be a valid string email address or an instance of Swift_Address.'
In the maillog we can read:
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Jun 13 15:33:42 mail1 postfix/smtpd[7867]: connect from unknown[10.2.1.198]
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 127.0.0.0/8
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 127.0.0.0/8
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.193/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.193/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.194/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.194/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.195/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.195/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.196/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.196/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.197/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.197/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.198/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.198/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 220 mail1.somedomain.com ESMTP Postfix
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_create: SASL service=smtp, realm=(null)
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: name_mask: noanonymous
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: Connecting
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: VERSION?1?1
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: MECH?PLAIN?plaintext
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: name_mask: plaintext
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: MECH?LOGIN?plaintext
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: name_mask: plaintext
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: SPID?7869
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: CUID?1
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: COOKIE?1d66640022b13972be6c0c83b520c519
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_connect: auth reply: DONE
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_mech_filter: keep mechanism: PLAIN
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_mech_filter: keep mechanism: LOGIN
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: watchdog_pat: 0x7f67acc8b180
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: < unknown[10.2.1.198]: EHLO web2.datacave.local
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-mail1.somedomain.com
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-PIPELINING
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-SIZE 15728640
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-ETRN
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-STARTTLS
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-AUTH PLAIN LOGIN
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_list_match: unknown: no match
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_list_match: 10.2.1.198: no match
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-AUTH=PLAIN LOGIN
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-ENHANCEDSTATUSCODES
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250-8BITMIME
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 250 DSN
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: watchdog_pat: 0x7f67acc8b180
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: < unknown[10.2.1.198]: AUTH PLAIN <deleted pwd>
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_server_first: sasl_method PLAIN, init_response <deleted pwd>
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: xsasl_dovecot_handle_reply: auth reply: OK?1?user=domainadmin@somedomain.com
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: > unknown[10.2.1.198]: 235 2.7.0 Authentication successful
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: watchdog_pat: 0x7f67acc8b180
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: smtp_get: EOF
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 127.0.0.0/8
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 127.0.0.0/8
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.193/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.193/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.194/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.194/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.195/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.195/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.196/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.196/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.197/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.197/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostname: unknown ~? 10.2.1.198/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: match_hostaddr: 10.2.1.198 ~? 10.2.1.198/32
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: lost connection after AUTH from unknown[10.2.1.198]
Jun 13 15:33:42 mail1 postfix/smtpd[7867]: disconnect from unknown[10.2.1.198]
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Interesting is the second last line 'lost connection after AUTH from unknown....
No idea why!
Tom