Gmail's Smart Reply is a feature powered by machine learning which will scan your email and suggest three replies according to the content of the email.
If you like anyone of the suggested replies, then click on the one you prefer and it will appear in the reply email. Customize that email if you prefer and then hit the "Send" button to send the reply email.
Keep in mind that the suggested replies will not be shown in all the emails. Also, note that you'll get better replies when you use this feature more.
If you don't like this feature, then you can turn it off on Android, but not on the web version. Here's how you can disable this feature:
That's it. The Smart Reply feature is disabled successfully.
If you wish, you can enable it at any time just by selecting the "Smart Reply" option.
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How to Disable Smart Reply in Gmail for Android