Sending with ISPs does not require username/password details, just the correct SMTP server name, which isn't always that easy to locate from ISP help pages, but some ISPs still reject use of any domain name which is not being hosted by them.
It protects against spam being sent out to spoof some other legitimate user. Hence my ISP (landline or mobile) would probably block me from sending out e-mail claiming my e-mail address is [email protected] as it would get the ISP bad publicity.
It's really to protect the ISP. Now, some are more open (or clear) about what is possible and they may require the customer hosts their domain / website with the ISP (sometimes at high cost) to be 'allowed' to use that domain name when sending.
Asking the ISP will sometimes get nowhere as they can either say 'hey, if you are running a business then you should be paying more for a business account' or 'unless you host with us, our servers will not include your domain and your outgoing mail will be rejected'.
If you had been using a normal PC or laptop, I would have suggested using Telnet to attempt to connect to your Streamline SMTP server. It is probably not possible with your mobile.
However, when last I saw complaints about not being able to send mail via the ISPs network, the problem was that the request 'seemed' OK (you would enter a command at the MS-DOS command prompt like:
telnet smtpserver.streamline.net 25
(where smtpserver is the correct server for your domain name)
If the response came back as
220 servername.streamline.net ESMTP
you would be connected OK to the Streamline server. However, on Orange/Freeserve it might say something like
220 externalsmtp.freeserve.net ESMTP
In other words, despite requesting an external server, the connection was being forced onto a particular freeserve system (and when it went down it was given lowest priority).
You probably don't have the option of telnet from your phone, so it is a bit academic, but maybe someone with Orange who has a different mobile can use Telnet.