Why are these email addresses rejected/

I signed in as administrator to send an email to our 13 club members. These 2 email addresses were not delivered per the following messages sent to me.

marooned@charter.net appears to be an invalid or non-working recipient email address. (No advertised SMTP servers are accepting mail.)

tberrys@charter.net appears to be an invalid or non-working recipient email address. (No advertised SMTP servers are accepting mail.)

I successfully sent the emails to both addresses using Yahoo mail.

Any ideas about why they were rejected by ToastHost?

it is the other way around charter.net is refusing email from FTH.

Please have your members add server@toastmastersclubs.org and agenda@toastmastersclubs.org to their approved senders list.

What is going on here is that we test each email address before each email send. While you might be inclined to think that a working email address would continue to work for us, our real-world experience is quite different. I am guessing that the charter.net email addresses probably used to work, but then charter.net changed some email policy (or spam filter) on their end. In fact, this is a good example of why we continue to test email addresses prior to each send.