1- Always start by checking your mailbox status on the server. To do this, visit: Access to WEBMAIL. You should see this:

Once logged into your mailmox, check available space. Sometimes, the trash can have lots of old emails, select “Trash” then click the settings button just below to “empty trash”.

2- Verify that you have the correct server settings. You should see this on a Mac:

On PC it could look like this: