This is usually caused by an environmental issue. Below are possible reasons and guidance on how it can be resolved:
Users(s) do not have read/write permissions to the designated e-billing path. You will need to get your IT team to check the permissions of the folder in question. If the permissions are missing then they must be granted.
Your Safe Mail Service has stopped working: To fix this you must restart your Safe Mail Service. Once you have done so, you will need to go back into Pay & Bill and re-try all emails that have failed via the email log.
The credentials for the account that runs Safe Mail Service has changed: You will need to ask IT to update the credentials against Safe Mail Services within Windows Services. Once this has been done you will need to activate the Safe Mail Service. You will then need to go back into Pay & Bill to resend all emails via email log.