[Evolution] Unable to retrieve message.

[Fedora <fedora rovoreed com>]

I have a problem with Evolution Mail (version 3.16.5) that is steadily getting worse.

I get lots of (typically) ...

"Unable to retrieve message.

Cannot get message 1445510505.6156_169.ceri from folder 
/home/mike/.local/share/evolution/mail/local/.House.BT Bills: No such file or directory"

If I click away to a different message in a different folder, then come back to the email giving this 
message, then the email appears OK. So now having read the message, if I shut down Evolution, and restart, 
then an email that was giving error, appears again as unread, with the "Unable to retrieve message". Also If 
I delete one of these messages, it simply reappears on the next start of Evolution.

I have tried a backup and restore. Makes no difference.

At this point I guess I should say I've got about 15 years worth of emails here - about 12Gb in 
~/.local/share/evolution/mail

Has anyone got any hints or tips for sorting this out ?

Thanks