You might want to look at existing GPG solutions for postfix, Exim, etc. The gpgit perl script is a filter that encrypts the message body with the public key of the sender. I think it would “just work” with a filter hook at the delivery stage and the ability to pass the recipient’s email address on the command line.

https://www.grepular.com/Automatically_Encrypting_all_Incoming_Email

http://kacangbawang.com/encrypting-stored-email-with-postfix/

https://gitlab.com/mikecardwell/gpgit

-- 
John Wilkes

One of the advantages of being disorderly is that
one is constantly making interesting discoveries.


On Jul 6, 2017, at 3:10 PM, surgemail-support <surgemailHIDDEN@t@netwinsite.com> wrote:
> 
> We will look into adding a setting to allow something like that at the delivery stage.
> 
> 
> 
> ChrisP.
> 
> On 6/07/2017 5:37 p.m., John Wilkes wrote:
>> I was hoping to implement encryption of incoming messages with a filter that encrypts the message body with the recipient’s public key, but that requires something like g_filter_cmd at a later stage, before delivery to a single recipient’s inbox.
>> 
>> I imagine that this has been discussed in the past...
>> 
>> 
> 
> -- 
> I'd really appreciate it if you could take a moment to like us on FaceBook, thanks heaps! ChrisP.