Sent mail not sent because Opera not contacting outgoing mail server Sending e-mail failed. Mayb

This seems to be a problem on Opera forums since at least 2004.
I am based in Australia. I use Vodafone as my ISP and a "pocket wifi" as the modem for my mobile broadband.
I have a MacBook Pro using Mac OSX 10.6.8.
I use Opera as my browser. I am running Opera 11.52. Build 1100
I have an email account with my company the mail server is: mail.dstgroup.com.au
I use a hotmail account (both via a web based portal and also routing it through Mac mail) and it has not had any problems.

The incoming POP server = mail.dstgroup.com.au
Port =110 TLS =No
Authentication =Auto (Plain text)
username = nigel.morris@dstgroup.com.au
Password = not displayed

Outgoing SMTP Server = mail.dstgroup.com.au
Port = 25 TLS= No
Authentication = Auto
username = nigel.morris@dstgroup.com.au
Password = not displayed


I have been running my DSTGroup email account through Opera mail for a few months quite successfully and Opera mail set up the account easily.

Three days ago, with no changes on my system or the DSTGroup system that I am aware of, the outgoing mail from Opera Mail stopped being sent and accumulated in the Outbox. Incoming mail has remained fine all the time. The messages about connecting authenticating etc went through fine for the incoming mail items, but when it went to send the message in the status bar just sat at "Connecting" and after a while the error message came through. (Incidentally, if I clicked on the link to open the error message box, it initially looked blank, but if clicked on the "error message" rather than "warnings" the message showed up. That's not terribly helpful showing a blank error message box as the default. )

If I logged into my DSTGroup mail website I could receive and send mail perfectly well from the mail website. So the error appears not to be the sending of the mail, but the contact between the Opera mail system, through Vodafone's ISP service to the mail server at dstgroup.com.au.

I checked the forums, mainly on the error message I received which was:
"Sending e-mail failed. Maybe the SMTP server is not set up properly? E-mail queued."
No other help, not a very helpful error message !!!!!

I went through all of the suggestions I could find on the forums:
I double checked the spelling of the mail server, my user ID and my password. They were all correct.
I checked with DSTGroup and they said "No changes"

None of the suggested solutions worked: like changing Port from 25 to 587, changing the Authorization setting.
As a back check, I tried adding my dstgroup email account to my MacMail system, which was working fine for my hotmail account, and the MacMail system said it couldn't contact the mail.dstgroup.com.au outgoing mailserver.

However, I read one forum comment (from 2004) that is outside my control
"Some ISP providers block the use of 3rd party outgoing mail to reduce the amount of SPAM that is sent from their network. You can check your latest ISPs instructions on this issue."

I have gone to the Vodafone website and found out the Vodafone SMTP server address is "smtp.vodafone.net.au".

When I switched the outgoing SMTP server from mail.dstgroup.com.au to the Vodafone SMTP server, and switched Outgoing to Authentication = None it appears that the mail is now going out.
I have sent a request to Vodafone asking for clarification to see if they know/will admit anything.

I'm still not sure about setting up my Opera mail outgoing server if I use the Vodafone server.

What settings should I use to use the Vodafone SMTP server as my outgoing mail server in terms of Port and Authentication
Do I still use Port 25?
The Authentication options are:
AUTO
AUTH CRAM-MD5
AUTH LOGIN
AUTH PLAIN
NONE

Is there likely to be a setting in my ISP Vodafone's system that would block my outgoing mail from Opera Mail to my mail.dstgroup.com.au server?

What should I do to avoid future mail not going out?

Why would my mail suddenly stop going out through Vodafone to my mail server, after working fine for months, without me having done anything to my system?