Email

Device or computer?

OS

Troubleshooting Apple Mail

What does Mail/Window/Connection Doctor Show? If the server is red, select it and look at the Show Details box.

Mail: Mail troubleshooting 

Mail: If you can’t receive messages.    

Mail not sending or receiving messages after updating OS X.      

IOS

Have you tried deleting the account(s), restarting the device, and then adding the account(s) back?

Mail – Setup on a device    -

Mail - If you can't receive email on your iPhone, iPad, or iPod touch    *

Did you try the OS troubleshooting I posted above? Have you tried deleting the email account, restarting the computer, and then adding it back? Are you running any anti - virus programs? Using a VPN?

If the problem is still there, try booting into the Safe Mode using your normal account.  Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer or by using the developer’s uninstall instructions. For each disable/delete, you will need to restart if you don't do them all at once. 

Safe Mode - About.    

You are welcome.