There have been reports of people with BT email addresses not receiving emails – and not just from my website. Occasionally it also affects people with other email providers. Huge thanks to Nick L. of Hexham Orpheus Choir, and John C. of Crescendo, who have quite independently come up with a solution.
Nick has kindly written up very clear instructions for BT addresses, but the principles also apply to other email providers. I give him thanks for letting me provide a link to them: