[X-Newbies] Mail deleting unread messages in the Inbox

[Richard Bienvenu]

For reasons I cannot understand Apple's Mail app has taken to deleting 
_all_ of the messages in the Inbox when I click on one of them. Has 
anyone experienced this odd and destructive behavior?

I perhaps should add that I am also getting mail via Thunderbird. 
Perhaps "Mail" is deleting the inbox because it "knows" that these 
messages are available in the Thunderbird inbox? But how could Mail know 
this?

Thanks,
Richard Bienvenu
U. of Mo., Columbia