This improves debugging when messages get lost in any direction during the invite process.
The general idea is to provide feedback about internal errors that may already be present in the logs of either the server or the client.
The most recent related user report was the following:
It also happens to me sometimes that I fail to onboard a potential Delta Chat user (who refuses to install or use chatmail) because they failed to find out why their self-managed classic mailbox host was not behaving properly as it was fully operational in other ways. Invite messages were bouncing one way in that specific case.
I also believe published chatmail relays in Chatmail: Relays should be periodically tested by an automatized process (such as it happens with R packages published on CRAN, for which there are regular CRAN checks) which would unpublish those failng such as the one causing the issue.