Time for a story;
Once upon a time someone asked me if they could send email when they were on holiday via Wifi.
Seemed a logical question: "Yes, wifi is just another way of accessing the internet".
The came back from holiday very disappointed, the email would not send but it did receive.
I looked in to the issue and sure enough it is common for ISPs to block SMTP that does not come from client connected directly into there systems.
Some offer alternate secure routes to send email but you have to know how to set them up.
The network in question was Demon
who are well renowned for their technical prowess.
And there on their help pages was how to send email when not connected to Demon.
But it did not work and every permutation under the sun of security / post hostname settings would get it anywhere.
Demon do have a Live Chat
support option and I suddenly realised that I had exhausted all the settings possibilities the problem had to be elsewhere.
After trying alternate email clients and machines I know what to ask the Live Chat
"Can the POP3 password be different to the SMTP password ?" remember POP3 (incoming) mail worked fine and sending from inside Demon, i.e. from home worked fine.
The answer was "yes they can be different", it never occurred to me that the sending mail password would be different to the receiving, many ISPs I have used in the past do not even have the option.
Armed with this crucial piece of information I now setup the SMTP and it worked.
Due the power of the following information it is provided in a separate post [ see above]