The error message indicates a policy restriction is the issue, not a technical one. Some possibilities: - change of email signature that violates policy - using different email id in account than in SMTP server settings.

Can you explain what you mean by change of email signature? Do you mean the server name (it is correct) or the server description or User name, those are the only things that I see to configure. She only has one email id and it does match the SMTP server settings. I do think this is the correct track. Seems like something does not match up. Did I mention that this started happening after the 115.9.0 update, not to mean that the update was the cause of things not working but rather could the update have changed those settings or the way they are put together?

By 'signature' I was referring to the text content and any possible attachments. For example, there are some antivirus products that force a small advertisement attached to messages. My only other thought was whether her email server setup might be using your id and not hers. IONOS posted in january that all accounts must have SMTP email id match the email id of the email message. Otherwise, I don't know what policies they are considering. But do review her account settings for details, such as use of complete email address and not just the prefix prior to the @ sign, and that her SMTP settings reflect her email address. Beyond that, I don't know their policies. If available, that might be a question to them - a list of policies they enforce. And they do owe you that, since her emails are being rejected for policy reasons. I wish you well in this.

Finally got clarification from IONOS. We created a second account (my e-mail) on my wife's machine and it would not send to that account (go the error message described earlier). Configured Outlook on that machine and was able to send from my wife's account. So there seems to be something wrong with Thunderbird on my wife's machine. They looked back at the error message and it basically says that my client is not in compliance with RFC 5321/5322. But we know that is not true because everything works on my machine (PC running Windows 8 - my wife is on Windows 10). So my assumption is that Thunderbird was somehow corrupted or something else is corrupted that causes that RFC violation. I have removed her e-mail account from Thunderbird then downloaded and installed Thunderbird again and added back her e-mail account and we still get the same error message when sending. Any ideas.