First thing to do would be to verify that your Account Settings in Thunderbird are correct. Here's a link to their email help page - they even have a section for Thunderbird!

https://www.optus.com.au/support/answer/using_an_email_application_with_optusnet_email_1434

Please perform a couple of checks on computer:

VPN Some Anti-virus software now offer VPN and I've discovered that sometimes they auto enable it. Servers do not VPN as it hides IP address, so server thinks you may be a hacker. Switch off VPN. Please confirm you do not have any VPN switched on.

FIREWALL Access the Firewall - I presume you have not installed any Anti-Virus so will be using Windows Defender. Please Confirm - Thunderbird is set up as an allowed program. That will ensure Firewall is not blocking any required ports. Please post/upload an image file showing your current outgoing smtp server settings.

Anti-Virus Q: What Anti-Virus you are using? Are you allowing scanning of outgoing mail and scanning of any file that gets opened? It's possible the Anti-Virus is delaying sending because it has control over the email and it's scanning both it and the file where it's saved in Drafts and the file which is being opened as location to store sent mail and in the meantime the server timed out. Suggest, switch off scanning of outgoing mail to see if it helps. Also suggest, if you allow scanning of files in your Thunderbird profile, then make the Thunderbird profile folder exempt from scanning.

Check 'Reply-to' In 'Account Settings' In left pane select the name of account email address On the right look for 'Reply-to Address' Do you have any email address in this area? if yes, Is it an email address which your server allows?