Did you know that UK Retailer Tesco offered email service? Me neither, but I'm not their target demographic. I assume they did or do offer internet service and that the email service perhaps went along with that.
Anyway, as of June 27, 2018, the email service at tesco.net is no more. Users are advised that if they have set up email forwarding, it will continue to work until October 10, 2018. If users had not set up email forwarding before the shutdown on June 27, they are out of luck.
Tesco has published a FAQ that you can find here.
What should senders do? It sounds like there's a chance a few tesco.net users are still receiving mail by way of having it forwarded to some other address. If you want to try to keep in touch with as many of those people as possible, it might be wise to target tesco.net subscribers with a "please update your email address" campaign before October 10th. There is no way for senders to automatically know what the updated address for a tesco.net subscriber will be; the subscriber will have to choose to tell you.
After October 10th, it's time to stop sending mail to tesco.net, as that mail will not get through to any real people. And continuing to send to a dead domain can lead to you hitting spamtrap addresses, if they later repurpose the domain to become a spamtrap domain. Or if they let the domain lapse and somebody else picks it up.
H/T: Arnaud Clément-Bollée