M365 Integration for Web App

I'm developing a web application that needs to connect with Microsoft 365 to facilitate sending and receiving emails through SHARED inboxes.

Here's the current setup:
- One licensed M365 account and 99 delegated access accounts configured via IMAP.
- A proxy between my app and M365 has been established, but it's not functioning properly.

Key issues:
- Authentication problems
- Message sending issues
- Message receiving issues

Ideal Skills & Experience:
- Proven experience with M365 integration
- Strong knowledge of web applications and proxy setups
- Expertise in resolving authentication and messaging issues

I'm looking for someone who can troubleshoot and fix these issues efficiently. Let's discuss if you have ideas or similar experience.