if your iphone is not working, why ask here for a solution. Clearly the settings are not correct or you have other security issues that prevent mail working.

It appears the server should be ionos https://www.ionos.com/help/email/general-topics/settings-for-your-email-programs-imap-pop3/ is that what you are using?

I am also unable to send emails to the UK servers of IONOS. My wife has had this problem for months too, if not years. I just spent half an hour on the phone with IONOS support while we tried all different combinations of server settings. The settings work fine in Outlook.

Robin, what makes you think you have the same issue as the original poster here? and what are the settings that work in Outlook that do not work in Thunderbird and what is the associated error message. What you have said offers no clues for the original poster to fix their issue and no useful doagnostic information for a yseful reply to fix your issue. So I am not sure why you posted here.

Settings that work in Outlook, also work in Thunderbird, it is a funny fact of life that both clients have to offer the same credentials for the server to allow them to connect. The only exception to this is where Microsoft exchange is the server platform. Outlook uses a completely Microsoft proprietary protocol in that circumstance. Outlook is a truly terrible choice of a test bed to compare functionality. It is like comparing the times newspaper with a paperback novel. Both contain the written word and that is about where the similarity ends. Many antivirus vendors flog addons in Thunderbird to menage spam and scams that cause complete failure frequently post a Thunderbird update (monthly) Outlook updates what every 3 or 4 years is my experience so again even if the AV product is using the same technology in both it has heightened risk of failure in the product that has frequent updates because it has not been tested with the new version.

I am thinking your issue may well be related to antivirus products not 1and1 as you are not experiencing the same issue on an iphone. I have spent years listening to the same old tired issues about certificates and failed connections and lost emails and time and again it comes back to the same old tired antivirus products that have been messing up with monotonous regularity for a decade or more. The products are sold on consumer fear. They endeavor to engender a state of unease which they purport to "fix" usually by doing something entirely pointless but sounds good to the uninitiated like scanning email.

I am also trying to move away from the “new Outlook” client because I find it slow and unreliable. That is the only reason I have been using it as a benchmark.

As you probably know, IONOS and 1&1 are essentially the same service. In Thunderbird on Windows, I can receive mail from my IONOS account without issue, but I cannot send. The OP reports exactly the same behaviour on macOS: receiving works, but sending fails. While I am not suggesting that the underlying cause is necessarily identical across platforms, the symptoms are clearly consistent.

For comparison, on Android I use FairEmail, which connects to IONOS’s servers and both sends and receives with no problem. I previously used the IONOS email app but removed it because it became unreliable.

Regarding security software: I do not see why the stock Microsoft antivirus would prevent Thunderbird from connecting to IONOS’s SMTP servers, particularly as it does not actively scan email traffic. This makes me suspect the issue lies elsewhere, perhaps in the way Thunderbird interacts with IONOS’s SMTP authentication.

"Sending of the message failed. An error occurred while sending mail. The mail server responded: Authentication required. Please verify that your email address is correct in your account settings and try again."

As noted above, I had IONOS support on a screen share and we tried the recommended and every other combination of setting before she gave up. If there is more information I can supply, perhaps a diagnostic log, then please explain how I can do that.

thank you in anticipation.

I fixed it with the help of ChatGPT. The problem was that Thunderbird had not stored an smtp password but also was not asking for one. I had to delete the imap password, relaunch Thunderbird, then it asked for the imap password and, when I tried sending an email, the smtp password. Now both passwords are stored in the passwords manager. Seems like a Thunderbird bug that it has a password setting for smtp, hasn't stored one, and isn't asking for one but just stalls on sending waiting for something to happen.