Cannot receive emails.

Since we have no idea what you have tried so far, to no avail, we can only offer suggestions that you may already have tried.

As such, let's start with:

• Is this issue with only the Apple Mail app or any email client that you use? ... or is this what you meant by "Other programs?"
• Can you receive email using the email provider's web interface?
• Does this affect all of your email accounts or just a particular one?
• Does this affect all of your potential senders or just one particular one?
• Do you use a VPN?
• Do you use a software firewall on your mini?
• Do you use any type of Anti-Virus apps on your mini?
• Do you use any type of app cleaners, like CleanMyMac?

One option is to run an EtreCheck report, and optionally, post it here if you need assistance with interpreting its results. If you are not already aware, EtreCheck is a diagnostics tool, created by a valued forum member, that greatly aids in troubleshooting difficult issues with Macs.

Ref: Using EtreCheck to Troubleshoot Potential… - Apple Community

For some reason this sometimes happens. Apparently your ID or password for receiving emails has gotten corrupted. The best solution is to first save any emails you have to a local email folder below. Then delete the email account in the computer. Next, create a new account carefully entering your confirmed ID and password. That usually solves the problem.