Why do I get this error - email delivery is temporary disabled?

Why do I get this error: email delivery is temporary disabled?

The error "Email delivery is temporary disabled." is specific to the following situations:

1. You have no active delivery server.
2. You have one/more active delivery servers but they are over the hourly quota
3. You have a valid delivery server but you have set some sending quota for the customers and that particular customer reached it.
4. You have a valid delivery server but the customer is in a group that has certain sending quota and he just reached it.
5. You only have delivery servers for campaigns (the Use for flag from the delivery server is set to campaigns).
6. Your customers are set in a group that is not meant to reset the sending quota for a fresh start once it has been reached.

Fixes for the above issues as they are listed:

1. Make sure you add and validate at least one delivery server and it stays active after you modify it (some changes to the delivery server require revalidation).
2. You have no option than to wait till next hour when the hourly quota is fine.
3. Check your Backend -> Settings -> Customers -> Sending settings. Maybe also reset the customer quota.
4. Check the particular group settings, the Sending tab is where you should look at. Maybe also reset the customer quota.
5. Add a delivery server where you set the Use for flag to All.
6. Edit your customer group and in the Sending tab, set the "Action when quota reached" to "Reset the counters for a fresh start". Please note that you might not want to do this as it means once your customers Hit the quota, it simply gets reseted, which beats the whole purpose of having a quota in the first place.