What do u think of Protonmail?

I've just signed up for protonmail, and I've got 500MB of space, this type of email service is really new to me, I've noticed that every time I receive or send a message the space gets smaller and smaller, if I understand correctly once I've reached the space they've allocated me the account can no longer be used.
I thought it was drive space but no, I wonder how this type of messaging really works.

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