In the last few weeks I can't send an email list using Thunderbird on my PC or Apple mail on my IPAD to more than 5 people and it's not the client server or my security apps. For Thunderbird I get a "1550 error", even for a small list of 8 people. The 6th plus rest of the addresses always come up as not valid even though I can send a singular message message to each of the addresses. For OS I get " X...is not a valid email address and has been rejected by the server." 1) I've talked to server support and when I use their web email (a pain), everything goes through. I have enough space on the server. I'm supposed to be able to send a list with 99 people on it. They say this test proves it is Thunderbird's problem but I wonder why it is happening on OS with Apple mail, too. 2) I talked to my security company McAfee and when we deleted the app I still had the problem. 3) I deleted Thunderbird from my PC and added it back again and still had the problem. 4)The server company suggested that I talk to Thunderbird about the PC and for OS delete the Apple mail app completely , not just remove the bothersome account and then try with a new app. This seems scary to me, as I will lose years worth of collected address and contacts. Thanks if you can help.