If the customer has an onsite IT Department, they can always set up STunnel on a server or a special purpose workstation. It works as an SMTP relay which allows you to send email through it over regular port 25 and it will add the appropriate encryption when it sends it to the real outgoing mail server. Beyond MFP's it will also work with almot any kind of device that can send automated emails. We even use it in our office for our now ancient MP 171 for scan to email, as well as with some applications that can send automated emails.
We've also built small Linux VM's (either CentOS or Debian since their support is fairly long) for some customers, or shown them how to install it on a Windows PC. All that needs to be set in it is the mail server to send to, and a few other steps and it just works.
Microsoft also has recommendations for scan to email with O365:
How to set up a multifunction device or application to send email using Microsoft 365 or Office 365 | Microsoft Docs
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