Ted Harding writes:
Once dialled in to their ISP service, it's no problem to send anyone a message from (say) "Zaphod.Beeblebrox@cosmos.god.org" (not making this up -- I've done it as a test) by doing an SMTP dialogue with their SMTP server, with "helo" from some nonsense address. The only thing that would stop this getting through would be a block at the receiving end.
This is the way most ISP's SMTP servers work; they'll accept pretty much anything that comes from one of their subscribers. They can, of-course, use their AUP to ban you if you do this and someone complains.
Open-relaying would be if they accepted such email from someone NOT directly connected to them (ie by dial-up or ADSL).
This is usually a good thing, as it allows me to send mail "from" more-solutions.co.uk addresses through whatever ISP I happen to have connected to.
I'm re-p***ed off because I've just had a message to Salford University bounced because the BT dial-up IP address used at the time was blacklisted ("550-Your email has been blocked, because it was delivered to us from an address that may be a source of spam email" -- and no wonder!)
This is probably a different issue altogether; I suspect that you are not using the BT relay but sending direct?
Traditionally, SMTP servers would accept email from anywhere, for anywhere, relaying as appropriate. This is open-relaying and has been massively abused.
Most SMTP servers now only accept email from their subscribers (destinations anywhere), or for them (coming from anywhere). (For recipient purposes, "subscribers" means addressed to a recipient it can deliver without relaying.)
This, too, gets abused; you can bypass a lot of spam checks by looking up the MX record for a domain and sending email to it directly, not going through any proxys or relays to get there. Done from a freebie dial-up account it is very hard to trace.
Increasingly, therefore, SMTP servers will not accept email for their own recipients which has come directly from an IP assigned to a dial-up or similar account (regardless of any history of spamming from it). They'll only accept email relayed through your ISP's SMTP server, on the basis that your ISP has some control over you, and in the event of abuse either the ISP will block you (cancel your account) or your ISP will be added to the list of iresponsible ISPs from which email is blocked en-masse.
At this point in time, most SMTP servers (in my experience) fit into the middle category - ie they won't open-relay but they will accept email from anywhere if it is destined to one of its recipients. Some large ISPs (I think AOL is one of them) blocks email direct from dial-up accounts, although this seems to be a large scale experiment where sometimes it works and sometimes it doesn't; maybe they're feeling the water.
The full text of the 550 bounce would probably make the situation clear.
FWIW: The best solution is to get an authenticated SMTP account somewhere, ideally one you have control over (eg I use my hosting server and have complete control over who can use it) and always send through it. That way the same setup will work whether you're dialled up to an ISP or directly connected through a LAN (otherwise you're forever changing SMTP server settings). Because you have complete control it should never be used for spam so should never get blocked by anyone.
As an aside, with BT Openworld ADSL they do actually limit the domains you can send through their SMTP servers as a customer, which without jumping through hoops means you can't send email from you@example.com unless example.com was registered through them; only your @btinternet.com address will work. This is major cause of grief, and to my knowledge only BT do this.