This happened to me on my iPad, and first noticed around 15 April 2024. In the past, as in every since the service was available, Optusnet has allowed sending to the outgoing POP3 mail gateway without requiring a logon. It's possible that this process has been changed to require [tougher] authentication (and, as with other changes made by Optus over the years, I can't find any indication that something was about to be broken). And I'm well over bothering to try Optus support.
I appear to have fixed it by using the following
* going to iOS settings/Mail/Accounts
* select the account from which you are trying to send mail (typically named as the email address)
* Outgoing Mail Server
* select mail.optusnet.com.au (should be ON, and I have others OFF)
* settings
- Server is ON
- hostname is mail.optusnet.com.au
- Username is your email address (in form of [something]@optusnet.com.au
- Password is your password for that email address (same as incoming)
- USE SSL is ON
- Authentication is Password
- Server Port is 25 (I allowed this to default and it seems to work, despite SSL being on)
When I clicked "done", the server was confirmed and my next mail attempt worked - for the first time in over a week.
Your mileage may vary.