Just yesterday (2nd Nov) I became unable to send from Pegasus (using latest version).
Tonight I got an email from Bigpond saying they noticed I can't send, and the solution is to "verify and update my email settings (eg. Outlook) to authenticated mode."
How do I do that in Pegasus? If I can't find a solution I will have to change email clients, which I dread doing after decades in here.
Thanks in anticipation,
Bill Purcell, Brisbane, QLD

Not sure exactly what they mean by authenticated mode? The instructions don't seem to show Oauth2 requirements.
Pegasus does have some option that you might check
Tools / Internet Options / Sending / Edit
Under General what port are you using (465?)
Many years ago one could use port 25 to send email with no requirement for user id or password, but hackers used this to send spam, so options to authenticate were added, and usually only people on local IPs could use port 25, and now even that is blocked by many ISPs.

Under security tab option what settings do you have?
Never or via STARTTLS or via direct SSL connect

Then under SMTP Authentications there are some other option.
Question. Do you have any of these set?

1. Do a prior POP3 login. Some ISPs would require doing a POP3 login with userid and password before allowing the SMTP connection?
2. Log into SMTP server with user id and password (Setting I use)
3. Log into SMTP using POP3 definition

So, trying one of those setups for authentication might work if they are not setup.

Looking at https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings
it is shows telstra.com for servers addresses, so not sure what smtp server address you are using?
Also, has
Username: Your Telstra email address - ending in ‘@bigpond.com’, ‘@bigpond.net.au’ or ‘@telstra.com.’

On my setup, I only enter userid without @server info but some ISPs might need domain of email?

Don't know if any of that might make it work or not. If they are looking at an Oauth2 setup, I don't know how that would work.
Might want to try and send and create a log file and see exactly what it reports.

Tools/ Internet Options
Under General options at bottom click on create internet session logs.
Then try to send and email.
It will create files under the TCPLogs subdirectory in users mail folder files for POP3 and SMTP session. Might provide more details.
Make sure not to post any password or sensitive lines from log.

Hope some of that might help.

Many thanks for your comprehensive reply, Mike.
To answer your questions:

Under General what port are you using (465?) I'm using the Pegasus default- 25

Under security tab option what settings do you have?
Never

Then under SMTP Authentications there are some other option.
Question. Do you have any of these set?

No, none of them are set.

Do a prior POP3 login. Some ISPs would require doing a POP3 login with userid and password before allowing the SMTP connection?
Log into SMTP server with user id and password (Setting I use)
Log into SMTP using POP3 definition

Haven't ever needed to, and don't know how I would if I had to.

Looking at https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings it is shows telstra.com for servers addresses, so not sure what smtp server address you are using?

mail.bigpond.com

Also, has
Username: Your Telstra email address - ending in ‘@bigpond.com’, ‘@bigpond.net.au’ or ‘@telstra.com.’

As per my Username in here: bileo@bigpond.net.au
Only those who have been with Bigpond for over 20 years have a "net.au" ending- everyone else just has ".com"

On my setup, I only enter userid without @server info but some ISPs might need domain of email?

Hasn't been required before.

Tools/ Internet Options
Under General options at bottom click on create internet session logs.
Then try to send and email.

Thanks- I set this.

Then I tried sending an email to my other three email addresses (at yahoo.com.au and gmail and Hotmail). They all received it and were able to reply and be received.

It would appear just opening all the Settings tabs and closing them either triggered something, or more likely, something was fixed at Telstra Bigpond.

If it all turns to worms again I'll post a copy of the TCP log

Kind regards,
Bill

As per instructions.
Outgoing server details

Outgoing server address: smtp.telstra.com
Outgoing server port: 465 (recommended) or 587
SMTP authentication: On
Encrypted connection: SSL/TLS on port 465 (recommended) or STARTTLS on port 587

Change port from 25 to 465
On security check the starttls
Under the SMTP authentication, I use the 2nd one for both my local ISP and my gmail.

Many thanks again. I've saved your solutions into a readily accessible text file.
Very good of you to follow up.
Bill

Just an added note: Sometimes ISPs allow smtp to local IPs that they have listed as local, but sometimes miss out on some, so if you get one of those IPs it doesn't work.
Also, if you setup a VPN setup, you will be using an IP address that doesn't match with local ISP. So, is probable best to use one of the SSL configurations on other than port 25. That way it should always work.