Hello there We have read the question We try to help

If I can ask you if you use a software or dark mode somewhere with your PC and, for example, in silent mode or protruding browsing, for example? These topics can bring you to other things than you can normally send a message to your recipient or just send it to third parties.

Try to disable it for the time being.

And see if this solution makes more possible again

Greetings Firefox volunteer

Hello there

Start your Thunderbird software

Relaying error code 500

Select Extra Account Settings Go nasr down select Outgoing Server SMTP Select your outgoing server can click on it and choose edit. Shelter in username and password.

Enter your complete username of your mailbox name here: ...@domain.net.com and enter the corresponding password for this.

You also check whether your athentication is properly filled in.

Be convinced that your SMTP configuration must be good and working. First and follow the explanation described above

Greetings Firefox volunteer

Hello there

If the problem is fixed Feel good and try a addon from us Thunderbird.

https://addons.thunderbird.net/en-US/thunderbird/addon/send-later-3/?src=search

Greetings Firefox volunteer