SMIME Encrypted Emails Not flagged as Encrypted

I am having an odd error that I can’t find any information on so hoping I can get some help here. I have setup SMIME encryption on the wordpress accounts with this plugin. The test messages work perfectly. They are encrypted and Outlook automatically decrypts them with the imported keys as you would expect. The issue we are having is that when a form is submitted, the main reason that this was set up for, the email is encrypted with contents being added as the attachment smime.p7m, but isn’t flagged as being encrypted. So Outlook (both desktop and online versions) just shows the email as having no content, but with an attachment that it won’t let you download, access, or even see the file name or type of. If I open the email in a different email client such as Gmail’s online client it doesn’t show signs that the email is encrypted, but I can see the smime.p7m attachment, download it, and use my private key to manually decrypt it so I know the actual encryption is happening successfully. I just need to know if there is anything I need to do to get the form submissions to be flagged as encrypted so it functions properly, or even if there is something I can do after the email is sent to add the smime flag so that email clients will recognize it.