hi all,
I am now two steps further:
1. Did another short test after installing the contrib wbl. And have included the fallback server name or ip address in the fields: qpsmtpd whitelisthosts,qpsmtpd whitelisthelo, qpsmtpd whitelistsenders and spamassassin whitelist_from. The messages were now delivered and this was visible in var/log/qmail/current.
2. The fallback server should keep a mail message for 5 days, this turns out to be 12 hours. The deadline for delivery had already passed.
My tentative conclusion: the fallback server settings should be revised. I expected that it should have been received without any whitelisting. This once worked, I should have tested this regularly. The time limit of 12 hours was short. Fortunately, a message of inability to deliver was sent to the senders.
My remaining question: Could I have seen a log other than var/log/qmail/current on my SME server that the delivery failed?
Best Gert-Jan