"We've had a recent policy change here, and we no longer offer followed links. It's hurting our reputation and being flagged by Google."
As Rand Fishkin said in his 2012 Whiteboard Friday, "Stop link building and start link earning." This context is the foundation of all communication with publishing partners.
Ask yourself what details would be worth including here. Is the detail crucial to the communication? Would including it potentially prevent the recipient from understanding something or from responding?
By answering these questions clearly and in this order, I'm giving the editor permission to close the email at any time with the information they need to know that this opportunity is not going to work. This is the best gift you can give an editor. It shows that you respect their time and will keep the door open for future opportunities. It's how to begin building trust in a long-term relationship.