How to train an ML to categorise email?

How would I train a machine learning engine to recognise a particular classification of email?

MonkeyLearn seems to have become enterprise-focused.

Obviously.ai seems geared to answering questions and in its own dashboard environment.

And then there's the more abstract question of how you would adequately train such a thing. I already have a Gmail label containing several thousand emails of the particular type. Is this sufficient for training? Does an ML also need negative examples, ie. several thousand more emails that are not of the classification?

Thanks.