Email Server Troubleshooting on Linux

I'm facing a critical issue with my email server where both sending and receiving of emails have come to a standstill. The server is running on Linux and it's not a mainstream service like Microsoft Exchange or Google Workspace.

I'm looking for an expert in email server troubleshooting, specifically on Linux platforms. You should have extensive experience with email server configurations and be able to identify and fix the problem swiftly. Please provide examples of similar issues you've resolved in the past.