So, following several suggestions I emerge thunderbird for testing, and the first thing I noticed is it does what people insist it doesn't do: creates redundant Trash and Drafts folders, both locally and (cardinal sin) on the remote server! So now in mutt there are all these redundant mail folders screwing up my carefully created IMAP structure. So, merely invoking thunderbird has created a mess.
Doesn't appear to be fixable... Hopefully I'm wrong and there's a trick to it? -- klaatu virada nicto