Jo

Your post is full of 'saying' and 'speaking' words. The Microsoft grammar checker, on the other hand, is working to relatively 'strict' written English rules - though you are able, allegedly, to vary the rigidity with which it applies them. Herein, IMO, lies a crucial difference. If you use the word 'which' in speech a lot, instead of 'that', you are probably committing no solecism. In writing, however, In the interest of being inoffensive to readers of business correspondence, the Microsoft thingy probably becomes a touch obsessive/compulsive about these matters.

For what it's worth, I tend to ignore the grammar checker.

cheer

the sunshine warrior