Undeliverable email via Outlook app on Macbook

Only recently, I have found that Bigpond emails being sent via the Outlook app on my Macbook are not being delivered. It is often hours before I receive an Undeliverable email message. (The emails seem to appear as an item in the Draft folder but disappear when I open the Draft folder. The emails are showing in my Sent items.). Up to the time of this issue occurring, I had no issues using the Outlook app on my Macbook and I did not make any changes that would have triggered this. It just started to happen.

I do NOT have these issues if I send the emails via webmail or via Apple Mail (on any device).

I am not having any issues receiving emails.

I have tried updating the Outgoing server settings (in line with the help information with Telstra and other online sources). I have tried re-setting the email account in the Outlook app. I have tried removing and re-adding the email account to the Outlook app. I have tried re-setting my Bigpond password. I have tried uninstalling the Outlook app and then re-installing (and re-adding all e-mail accounts).

I am currently running Sequoia 15.6.1. I am using MS Outlook for Mac version 16.100.1