Apple mail - unable to send emails (Optus is my provider)

I have an IPad Air 3 and IPhone SE 2020. For years, I have continued to receive all emails on Apple mail, however 3 weeks ago I found that (for no particular reason) I was no longer to send emails on Apple mail on either device. When I try to send an email it goes straight to the outbox, with a message "the connection to the outgoing server mail.optusnet.com.au failed. Additional outgoing mail servers can be configured at settings, mail, accounts". Mail is currently set up as a POP account, with server number 995.

I have phoned Optus 3 times and Apple support once. Optus was unable to fix the problem - stating that using Optus Webmail is preferred (which requires a login and password!) and their technical people are working on network issues incl. email. With both Optus and Apple, I tried different server numbers (e.g. 25) without success. When entering a server number (other than 995), a message appears " The POP server mail.optusnet.com.au is not responding. Check your internet connection and that you entered the correct information in the incoming mail server field". Even deleted then reset the account - again without success.

I welcome any suggestions to solve this problem.