I feel wil's suggestions are unreallistic for notaries. First, I'd like to dismiss two situations where email encryption is either already solved or not an issue.
First, some notaries do all their notarizations as employees, and all the signers are fellow employees. These notaries will use whatever encryption the company provides; if the company doesn't presently provide any, they'll have to wait for the IT department to provide something.
Second, some notaries do one-off notarizations for the general public, with little or no repeat business. There is hardly any need for email in these situations.
Mobile notaries who do real estate notarizations for title companies and similar organizations and other notaries with substantial repeat business, have more need for email, and encrypted email would be helpful. But the employees in the companies the notary communicates with are employees. In most cases they don't have the authority to purchase any email product or service. Even if it's free like GPG, most of them don't know how to install it, and even if they did, their IT departments probably won't let them install software.
About the only encryption facility that's widely available to employees who work in offices is the password protection of Microsoft Word documents. Microsoft Word does not provide for encrypting a document with a public key; in that program, public keys are only useful for electronic signatures.
Once the Word document is encrypted with a password, it can be attached to an email.