Is an outgoing gateway implementation acceptable? If so you have lots of options.
But perhaps you need DLP between tenants of a multi-tenant server. Can SmarterMail be configured to send inter-domain mail through the outbound gateway instead of direct? (I have never needed this feature, so I don't know..) If so, you have the same options. If not, you are limited to the existing options. I can imagine a complex way to use Declude to force inter-domain traffic through an outbound gateway, but it might be hard to maintain.
For any implementation, you need to decide what actions you want the product to support:
- Reject mail back to sender
- Quarantine mail for system administrator review
- Automatically edit the message to filter the problem content, then allow the revised version to be delivered.
- Force the message to be user-to-user encrypted via secure web relay (Zixmail equivalent), but let it be delilvered.
- Can you accept a cloud solution, or does it have to be on-premises, so that the sensitive data never leaves your internal network? If on-premises only, you are looking for an appliance-type design (which may be implemented as a virtual machine without any new hardware).
- Does the product allow me to configure exceptions for the messages that are authorized by company policy?
- Some products are only sold in specific geographies, so it also matters where you live and work.
Clearswift is very proud of their content-editting capabilities, but if the message has content that needs removal, I would not want it delivered if any form. So editting is not important to me.
If you only need user-to-user encryption, a Barracuda appliance is sufficient and relatively cheap. But I don't know how long they will be selling appliances. Their appliance design has been only minimally improved in the last 15 years, and their development is going into their cloud solution.
I have surveyed a bunch of email filtering products, but not for this feature. I can send you a shopping list if you send me a private message.