How to integrate to external users outlook emails

I'm building a SaaS that connects to customer's work/personal emails and listens to incoming email in the background (so they don't have to stay logged in) and runs an AI task on the email to give some data insights.

Some of the customers use Outlook as their email provider. How do I connect to a customer's Mailbox via Microsoft Graph? (NB: customers do not live in our tenant)

I've tried the Delegated User flow, but this doesn't work well for me as the key is short lived (1 hour) and I need to be able to access their emails even when they're not on the platform.

I've also tried the Application Only flow, but I keep getting a 401 status code when trying to access the email of someone outside my tenant/organisation (even if they're a guest).

Is there any plausible way to achieve this in Microsoft Graph? I know with Gmail it's pretty straight forward with their API.