...in the past I have used Symantec E-Vault, I have also been asked to look into Arcmail.
We have not even moved forward with a plan yet, but at this point we are shopping around for a good e-mail archiving solution.
We are using Office 365 on the Client end.
I would look to find a product that requires minimal administrative effort and that is easy for the end users to work with.
Thank you.