How to limit Gmail API and MSGraph email API to specific email subjects - django

Question

I've built a CRM webapp with Django for a specific lead heavy industry. It's working for both gmail and outlook users. Through MsGraph and Google API, the user is able to give authorization via Oath2 to the app to access their inboxes. The app then grabs and parses emails from various sources.
Each lead source always sends the lead emails with same subject. This makes the lead emails easy to identify from the users inbox. Unfortunately, the subject of EVERY email that comes in has to be searched to find the desired lead emails. Unfortunately, Identifying by sender isn't an option, and wouldn't change the issue. Each email would still have to be searched.
I have a couple of colleagues beta testing right now.
As I think about taking on new users that may be outside of my colleagues, I am starting to think the webapps unrestricted access to a user's inbox via the available scopes isn't the best approach for trying to attract new users. I would be suspicious of any 3rd party program wanting to access all of my emails, even if just searching for specific emails.
I use Google's watch() and MsGraphs subscriptions to do this while the user is offline. It doesn't appear that Google or Microsoft allow for any kind of message change filter based on what's in the subject line.
Are there any methods that I have not been able to find in either Google API or MsGraph documentation that would limit access to only the emails that meet the subject search criteria?
Would this even pass either of their security checks to get 'Published Status.'

Answer 1

Reading through the Google docs, it looks like you can set authorization scopes that limit access to just labels and basic settings. This should allow you to filter messages by subject and apply labels to those filters.
Of course, the subject filtering doesn't have anything to do with authorization. But fine tuning the authorization is better than allowing write access to an entire mailbox.
I would say in general, the more open the permissions are, the less likely you are to get approved. Google wants you to only have access to what you need to achieve the product's purpose, nothing more.
https://developers.google.com/gmail/api/auth/scopes
There definitely isn't a way to set custom permissions based on subject. In fact, I don't know many APIs in general that allow you to define custom permissions that granularly.
That said, it doesn't seem like you even need read access to message headers, let alone message body content, to achieve what you want to do in Gmail.
I assume Microsoft has similar scoping, but I'm not sure.