Messages to one recipient are being marked "undeliverable - looks like spam". This happens only on one recipient and when replying to a reply. His domain is btopenworld and it does not happen with other btopenworld addresses. How can this be stopped, please?
Thanks for your reply. Sorry, don't have a different e-mail account or a different device. Should have mentioned I'm using Windows 10, Firefox browser and Thunderbird e-mail, both current versions. Also, the problem seems to have cured itself but any ideas in case it returns (or something similar occurs) are much appreciated.