It wouldn’t matter to them really. Just look at how many people have gmail accounts.

They don’t even have to send the whole messages back to base. They could be categorizing your messages in to themes and sending that back to base as small category flags. Use that to build a profile on you and use those for advertising to you.

You mention something on the theme of ‘broken boiler’ in a message, that gets analyzed on the client in to a category of ‘interest in heating / boiler repair’, plus some adjacent categories based on your demographic. The categorization gets sent back and the next website you visit has an ad for British Gas boiler repair.