A few more good follow up questions were posted by an anonymous user: Do you know if the Mail extension allows the modification of the content of an email when composing or replying? When right clicking on the body of the mail (composer view), a contextual menu appears: is this possible to add an item in there with actions on the content of the email being composed? if one of this is possible, can anyone suggest some guidance?

I need to be able to encrypt and decrypt message attachment files (only attachment, not whole mail) on demand, so I need to access attachment data while sending and receiving email. With a mailkit security extension(MEMessageSecurityHandler), I can see a message security handler can get the full RFC 822 message data during composing or receiving. It can then parse the mime data to access the mail attachments. but can I just encrypt the attachment and not the rest of mail data? If I just encrypt the raw data for attachment and mark it as encrypted, Probably it would work fine if my decoder sits in the other end and decodes it appropriately?

We also see that our Finder sync extension does not run on its own in MacOS Sequoia. The OS should notice and load the .appex from plugins directory within the .app. But it doesn't do so. Running plug-in kit command (pluginkit -mAD -p com.apple.FinderSync -vvv) lists the extension but it is not enabled. If I manually enable it (pluginkit -e "use" -I "my plugin id") by using pluginkit command, then the plugin is loaded by OS and it runs fine.