When you start a mass mailing task in 1and1Mail, sometimes you will encounter a pop-up message: "List index out of bounds (8)".

List index out of bounds

It is usually caused by your modification (import, add or delete) of emails in the address books that are being used by the ongoing task.

To resolve the issue, please restart 1and1Mail and start a new sending task. Make sure there is no change on the address book when the task is ongoing.