BOAC and Osmosis - Very many thanks ... we have lift-off!!
. Your advice(s) plus a confirmatory call to Plusnet (God bless UK-based helplines!!) and all was revealed. Main problem was the inclusion of 'smtp' in the outgoing field - for Plusnet, it is left out!! Additionally the user name was incorrect as you suggested. All now working as advertised and SWMBO happily clogging the internet!!
