I feel this really depends on your requirements.
If this is for a business with less-than-technical users you may want to go something that is more managed and in that case I would make a strong argument for Microsoft's Office 365 platform. You can turn on encryption and the implementation is simple.
If you ran a security company then you'd definitely want to go with something less managed like Mozilla Thunderbird and some GPG/OpenPGP software, such as Enigmail.
It seems as though Virtru is just managing the encryption of O365 or Google. This is great for your folks, but the stumbling block of encrypted email has always been on the recipient's end. If their client does not make it easy, the email with either be ignored and/or deleted. Some platforms do not allow users to accept encrypted email, like some helpdesk platforms. There are always two sides to this coin. So many services try to make it "easier," but it is usually just for the sender.
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