I have tried the 'sledge-hammer' approach: delete all email accounts, uninstall Dekko2, clear residual Dekko files in .cache and in .config, reboot, reinstall, re-set up one IMAP account, and ...... same situation! [This is all on a brand-new (one week ago) installation of UT on a Nexus 5.]

PS Whilst rummaging through the file store I spotted folders relating to Evolution. Is this just some random hangover from Canonical's original ambitions, or is some version of Evolution available on UT?