During an initial Sync you may see the error Failed To Validate Domain. This is quite usual if this is an initial Sync.
Wait It Out
Microsoft may be taking longer than usual to configure the domains we need setting up.
Subsequent revisits to the Sync button in our portal should eventually resolve this but we have noted that on rare occasions, Microsoft can take several days.
If you feel you have been waiting excessively long, you can always ask Microsoft for support, quoting the relevant lines from the sync log.