We understand your trepidation regarding trusting the storage and management of your emails to a third party, but this is a growing trend among many businesses.
The ABA published two (2) very informative articles:
"Ethical Issues Implicated by Lawyers' Use of Third-Party Cloud Services," by Amelia Toy Rudolph, dated February 23, 2015.
"FYI: The Ethics of Online Backup Systems," by Erik J. Heels, dated October/November 2007 issue of Law Practice Magazine
Another good source of information is the Florida Bar Ethics Opinion 12-3, which states "Lawyers may use cloud computing if they take reasonable precautions to ensure that confidentiality of client information is maintained, that the service provider maintains adequate security, and that the lawyer has adequate access to the information stored remotely. The lawyer should research the service provider to be used."
While these articles and ethics opinion deal more with "Cloud Computing," the message, concerns and guidelines are the same. Before utilizing a particular software or third party company, perform due diligence and research to ensure that they are reputable and will provide the highest standard of security and technical support.
The Florida Bar Membership Benefits page provides information on RPost, a Registered Email Service.
Another Registered Email Service about which we have heard good things, is Absio Dispatch.
I hope this information helps you with your concerns. If you have any further questions, please do not hesitate to contact our office.