It's probably this. For any "open standard" there's going to be a few different interpretations of what that means across the board. Different services often ha

It's probably this. For any "open standard" there's going to be a few different interpretations of what that means across the board. Different services often have sightly different ideas about how their APIs should work, which causes weird edge cases. Supporting all clients arbitrarily without first making sure they all work is a headache for devs and is likely to result in crashes/data loss/etc. I'm guessing that the client was never designed to be a fully fleshed out classic email app. They probably have some baked in ease of use/set up features that they had to explicitly included on a per provider basis. Having a list of pre-approved clients that are guaranteed to work probably satisfies the grand majority of users while leaving these features maintainable.