As they came top, Proofpoint are waving the 2015 edition of Gartner's magic quadrant for the SEG (secure e-mail gateway) marketplace at:
https://www.proofpoint.com/us/Gartner-MQ-Secure-Email-Gateways
Well look who came second for vision, and near-equal for ability.
By contrast, the other options your customer is interested in are much further down the chart. The download is free if you don't mind disclosing / composing a few details, and makes for an interesting read. The specific pros and cons which aren't visible on the chart may be factors for your customer.
Leaving aside the report, my own exposure to Proofpoint is very limited (I've got vague recollections of a web-based secure message facility I'd sooner not have had to deal with as a web proxy admin, though that may have been someone else and CRES is hardly a joy if you're not already tooled up to receive it). By contrast, both Barracuda and Fortinet solutions can cause problems precisely because they are sold as unattended solutions. If they are correctly configured to begin with, something changes later on and they become reject-monsters. I know for a fact that some on the Websense / ForcePoint forums are grumbling about the botnet issue.
We recently acquired a new C300-series ESA and have no plans to move away from Cisco unless we end up forced into a 100% MS-cloud environment for e-mail. One of our suppliers has already gone that way, but ended up putting their MX to a smarthost. I'm keeping my ear to the ground for similar (substantiated) stories and the ongoing saga of MS cloud reliability.
