After Server down and failover in a cluster the following actions do not continue working :
While switching from Inbox to a folder: The Inbox of your mail file is open (and the preview pane is not open). You attempt to open a folder and the warning message appears. Failover does not occur.
Reply to an email in the Inbox with the Preview pane open: From the Inbox, you view a document via the Preview pane, and attempt to reply to it and the error occurs.
The database needs to be reopened and the action repeated.
But some end users who do not know to reopen mail files again, they always waiting or try again in the current opened db. Since domino support cluster failover, which should can failover automatically in these common scenarios for opened databases.