I use my ISP's email via client Palemoon, a platform based on Mozilla Thunderbird. Recently I encountered a problem with sending to multiple addresses. As soon as I add another line, the address above is removed to be replaced by the one just input.
Palemoon forum never replied to a question, so as a member of Mozilla's forum I did ask there, because both are almost identical. They did reply with suggestions and links, which I have visited. Everything tried has not cured the problem.
I like the Thunderbird based systems as they work well with Plusnet's email and its unlimited addresses. If I want to experiment with another email client, would you have suggestions, please?