Sorry, I am using Mozilla Thunderbird. Thank you

Use mail.optusnet.com.au on port 465, SSL/TLS security, authentication = normal password, User Name = email address.

https://yescrowd.optus.com.au/t5/Broadband-Telephony/Setting-up-Optusnet-Email-POP3-VS-IMAP/td-p/588566

Thank you sfhowes, tried using that setting but still unable to send messages. Have tried no security with ports 110 for incoming and 25 for outgoing but still not working.

Any error messages? Do you have an antivirus app scanning outgoing mail, or are you connecting through a VPN?

No VPN, error message says connection timed out. No AV scanning outgoing mail. Optus support not really interested because webmail works fine.

OK, but what is your AV? The best way to rule out any interference from startup apps is to run Windows in safe mode:

https://support.microsoft.com/en-ca/help/12376/windows-10-start-your-pc-in-safe-mode

Ok thanks, I use Bitdefender AV.- does not show any option to scan emails. Tried safe mode in windows also started TB with add ons disabled - no difference.

Can you add a screenshot with the smtp settings?

Bitdefender has caused problems in the past, especially the 'Advanced Threat Defense' module. You also have exclude the TB profile folder from BD scanning.

https://support.mozilla.org/en-US/questions/1275164

Thanks FfTh202009, the problem is with my wife's mother's email so I don't have it here but the settings are as previously recommended by sfhowes above. The problem arose after Thunderbird was auto-updated to the latest version, she has got nowhere with Optus. Can she revert to an earlier version without losing her emails?

What operating system she have on the computer? Also, did you tried the recommendation mentioned by @sfhowes?

She is using Win 7 still...I did try previous recommended settings but no luck with them.

Try searching on Google "How do I enable TLS in Windows 7?" and check on her computer if she has all those updates and enabled tls 1.2

Alternatively, you may downgrade to v68 (install over it) and for profile issue start tb with this command --allow-downgrade

When v68 is installed it creates a new profile without any current messages attached..doesn't connect with the profile in current version 78

You can start thunderbird with "-p" command - and select that old profile. It will mention you that you can not use that profile - select exit After that start again thunderbird with a command but this time with "--allow-downgrade"

Thanks for that FfTh202009, this worked as soon as I realised I had to disable automatic updates for TB first.