My first thought would be to check SMTP server and verify that you have authentication set properly. That is, the server may not be recognizing that you are a legitimate user.

The settings are all good, it's only sending and the Sending server details look correct

Somehow, the server is sensing that the email id isn't allowed or the authentication is not taking place. If you have more than one email account, it may be that you have the SMTP servers switched.

I am getting a similar error message:

Sending of the message failed. An error occurred while sending mail. The mail server responded: mf—505: Sender Policy — Relay Denied. Please verify that your email address is correct in your account settings and try again.

This occurs when I attempt to send a meeting request from Thunderbird Calendar. All e-mails, in and out, function properly. I can save a calendar event, to the calendar, but if I add any other attendee (e-mail address) to the event the previously mentioned "Send Message Error" appears.

I contacted my email service and they said it is not their fault.

Here is a basic summary of relay errors that may prove useful. Such errors are not caused by Thunderbird, but by the email settings set by the user. https://support.microsoft.com/en-us/office/i-receive-a-550-553-or-relay-prohibited-error-when-sending-email-messages-65b494a4-0a15-433e-829f-e73651e0245e

Nice read, if only I understood 90% of it, that may help. All was working fine for over 2 months then out of the blue nothing worked. I contacted my email server provider & then emails began working in and out, but calendar appointments still failed. I believed this was still their problem, but they told me it was my mail client. I have since tried all the SMTP settings I can think of to no avail. One fact I have not mentioned is that I am using aliases, but I thought if in & out emails to main account, two aliases and other people's accounts all work, why is it just calendar emails that fail and return this message? Is there a different set of settings for the Thunderbird Calendar?

Aliases work only when being directed to the real account. you can only receive as an alias, but not send. That is, the SMTP settings must be the real account. I admit, though, that I am not familiar with calendar settings.

Yes, I can confirm I have SMTP set to the real account. I still believe that it is something they have done at the server side, but I don't have the expertise to argue the point so I am stuck. Thanks for your assistance David

I have the same problem. I have one email account (with internode) which allows me and my family five email addresses: me@internode..., wife@internode..., son1@internode..., son2@internode.... These addresses have all worked perfectly for well over 20 years until the latest upgrade of Thunderbird. All addresses still receive emilas; I can still send emails from me@internode, but my family cannot send any more and get the error message "Sending of the message failed. An error occurred while sending mail. The mail server responded: mf-505: Sender Policy - Relay Denied. Please verify that your email address is correct in your account settings and try again." All email addresses use the same outgoing server.

I cannot find out how to fix this and ask that Thunderbird attend to it as soon as possible.

If the internode email addresses are true addresses, and not aliases, you can fix this by setting up different SMTP servers for each member of the family. Be sure to give a unique description to each SMTP server to eliminate any problems in assigning the right one to each family member.