That's a bit odd. The spell check works fine on my two Linux systems, so it can work if it wants to. Do you have a current dictionary selected and engaged?

If you send without invoking the spell check, does it all go through OK? I'm wondering if the hang is specific to the spell check, or maybe more generally linked to sending the message.

Thant's a good point, will reboot Mint & check. But...have Thunderbird in Mint point to the same default folder as my W7 data partition, so it reads all the same (common) data - or perhaps not? edit: I've checked and that was indeed the problem. Switching between W7 & Mint disables all the addons every time, including the dictionaries. Have to re-enable them each time I boot and Mint is then fine. Thanks for your help.

I stopped trying to share a profile after I found that some add-ons store absolute pathnames for their resource files. It was painful enough modifying a copied profile to fix up all the pathnames, without thinking about crafting a profile that made sense to both OSes.

I don't know how the dictionary is managed. You'd hope it would have stored a simple relative location with forward slashes and so both systems would be happy. But maybe Linux is being given a Windows path it can't parse?

Just looked at it here in Windows, and in the config editor it gives just "en-GB", so there is no pathname to worry about.