What happens if I receive e-mail when I am out of quota?

The message will bounce back to the sender and they will receive a message similar to:
The user to whom this message was addressed has exceeded the allowed mailbox quota. Please resend the message at a later time.
When you use 90% of your NetMail quota you will receive automatic messages from the system postmaster account indicating that your mailbox is almost full.
Your UWP NetMail account is over 90% full. Please delete some messages to free up space within your account. Remember that all folders stored within your UWP NetMail mailbox count against your diskspace quota, including optional folders such as Sent Items and Trash. If your mailbox becomes 100% full, all new messages will be returned to the sender. This message is automatically generated by the NetMail system, and will be sent with every new message you receive when you are using over 90% of your account diskspace. To stop receiving this message you will need to remove messages from your e-mail account until you have greater than 10% of your account diskspace available.
To stop receiving these messages you need to delete or archive and delete some existing messages until you have at least 10% of your available space free. If you are using a desktop client, messages marked for deletion may not actually be deleted until you quit the client.

Your available quota is displayed under the mailbox list on the left side of the NetMail web client screen.