Give it a go.

Using mkryptor is simple; three easy steps

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1 Install mkryptor on, or alongside, your email server
2 Send mkrypted email
3 Get on with your day job

Your clients simply open the mkrypted email using any compliant PDF reader. It even contains the attachments you added to the original email securely mkrypted inside. If your client can open email, they can open mkrypted email.