When composing messages, right click in the text area and select Language and make sure a language is actually selected.

Matt many thanks for your help but when I click on language box it only comes up with 'download more dictionaries'. Any further help please. j

sounds like you just have not got a dictionary. The download option is at the end of the list of installed dictionaries, so perhaps it would be a good idea to download one.

Sorry Matt this did not work, it comes up with 'Dictionary already downloaded' but it still does now accept whatever I do. It actually works on this site as I am typing now but does not work on Thunderbird. Please advise if you can help. Many Thanks John

Sorry Matt this did not work, it comes up with 'Dictionary already downloaded' but it still does now accept whatever I do. It actually works on this site as I am typing now but does not work on Thunderbird. Please advise if you can help. Many Thanks John

It works on this site because you are using an Internet browser like IE or Firefox and those are separate programs from Thunderbird and you have a dictionary loaded for it.

When you go to the options page Tools-Options-Composition-Spelling is there anything specified in the Language field? If not you need to load a dictionary there.