What is happening with your emails, is that recipient's email servers can see it is a Namesco email address and is being sent by a BT email server.
Other email servers will then believe they are spam emails as they have come from a location they should not have. We highly recommend using our Authenticated SMTP server to send.
This for a mere £21 pa. I did used to have it, it made no difference, and BT is STMP anyway, so I cancelled it.
In case it's not obvious to you, the point is not about it being an SMTP server. SMTP is just the simple mail transfer protocol - implementing the basic rules for sending out mail that all servers have to follow. The focus is on the mismatch between mail that clearly is sent by a BT server, and yet claims to come from a Namesco address. That discrepancy is often a sign of spam, even though there are plenty of legitimate reasons for it to arise as well, yours being one.
This is where the Internet comes in. It's not a single entity; it's an Inter-networking of separate systems, each with their own systems managers, who get to decide what mail they will and will not accept. It's perfectly legitimate for one systems manager to reject mail with the above mismatch, and another to accept it. They are running their own systems, and are accountable for preventing virus and spam attacks. And sometimes the amount of rejected spam can overwhelm amounts of legitimate mail.
The authenticated SMTP comes in because it's foolish for an SMTP server to accept mail from just anyone. It would be asking to get used for sending spam. So, typically, BT will only relay mail sent from BT customers on BT connections, and so on and so forth. But Namesco probably don't provide Internet connections. So, they require your mail client to provide a username and password (authentication) when using their SMTP server to relay mail.
In theory, therefore, Namesco are offering a sensible solution that means that your mail will come from a Namesco SMTP server and a Namesco address, and other systems managers will happily accept it. In theory. But your experience suggests that they might not.
POP3 (Post Office protocol) is usually used for receiving mail, as opposed to sending it. Lots of stuff about email is like the real mail. Different rules for sending and receiving is one of those things. Receiving real mail, you wait eagerly by your front door for the sound of the letter flap. Sending it, you walk to the post box, and only the sender needs a stamp.
