Was that in UK by any chance? If so, take a look at Data Protection Act - especially this:
Personal data shall not be transferred to a country or territory outside the European Economic Area unless that country or territory ensures an adequate level of protection for the rights and freedoms of data subjects in relation to the processing of personal data.
If you want to be ok with the law, basically you cannot store your emails (which at some point will definitely contain personal data) in an unknown location. Gmail storage qualifies as an unknown location for all we know.