>What can we do with the value of 'messageId'? In particular we wonder whether this might be useful if the mail was sent, but a mailer demon replies (after some time) with an "undeliverable" error.
This is the unique id returned by the mail server. I suppose this would primarily be used to troubleshooting/tracking scenarios. This isn't automatically persisted or anything on the HANA side. It would be up to your application to use/store it as you see fit.
>Our application runs on several systems in different landscapes (dev, cons, cloud, hcp, ...). How can we find out, which (and where) the SMTP relay instance is, and which routing rules are configured there?
I'm not sure I understand this question. You have to configure your SMTP server/relay address in the XSAdmin tool. This should tell you where the SMTP server/relay is. That's all we really have from the HANA side - just an address and authentication information. Everything else would be handled from the SMTP server/relay side.