There is no built in auto correct feature in Thunderbird. There has been an addon for doing this, but when I have used it previously I have found it counterproductive. I am pleased and surprised to say that it now appears to be actively maintained, so maybe if it gives you trouble the current maintainer will be willing to improve it. https://addons.mozilla.org/en-GB/thunderbird/addon/mms-auto-correct/ The idiot autocorrelation tool on the Android tablet I am using right now insisted that I was looking for "mum's auto correct" but at least it let me enter what I wanted to say. The Thunderbird add-on isn't quite so amenable about being browbeaten into compliance. :-(