Do you get any sort of message telling you why the send failed? What is it?

I am sorry. I should have mentioned the notice in the first place. The notice said: SMTP.optusnet.com.au timed out. Try again. Thank you for helping. Anna

You are not alone with Optus. I closed my account with them more than 25years ago, but they just keep popping into these forums as someone who has customers that struggle with the arcane settings and security they use.

Do you have an antivirus that scans outgoing emails? Optus uses a very non standard port 25 to submit mails. The rest of the internet abandoned that about the turn of the century and left port 25 for server to server communications. Now we are seeing antivirus products and software firewalls blocking outgoing connections on port 25 as probably malware. You might also want to try with no email scanning at all. (defender does not scan email)

They still put this in their web site. https://www.optus.com.au/support/answer?id=1434

   Description: Optusnet SMTP server
   Server Name: mail.optusnet.com.au
   Port: 25
   Connection security: STARTTLS
   Authentication method: Normal password
   User Name: username
   Click OK

So if your settings are correct as shown above I would suggest you look at your antivirus blocking outgoing connections on port 25.

Hi Matt, Thank you for your advice. I changed the server name. Entered my password and now it seems to work. No idea what went wrong in the first place. Never mind. I am really pleased that it works. Thanks again. Anna