I am unable to send emails from a POP3 email account, though I can receive them. I have tried using different ports for the outgoing smpt server: 25, 587 and neither works. I have contacted my ISP provider who is not blocking port 25 (which was suggested)

What speeds do you get on the BT WiFi

Speedtest.net, use the same test for both.

The problem is that the server doesn't like the way your computer is identifying itself:

You: EHLO macbookf0:b4:79:21:d5:0c.home

Server: 501 Syntactically invalid EHLO argument(s)

This seems to be an issue specific to BT Homehub routers. There also seems to be a way to change the settings of the router to avoid it:

OSX 10.6 Mail SMTP fail (fine on Entourage, iPad and iPhone)

I can't tell you specifically how to change that setting because I know nothing about the device.

I do not understand why the Home hub works fine for this macbook pro to talk to the icloud mail and gmail smtp servers, just not when it's talking to the Acorn output smtp server.

Because that server is configured differently than the others. The rest has to do with the peculiarity of your router.

The BT wifi community works by users allowing a small proportion of their Homehub bandwidth to be shared with other BT wifi members. The chances are that the BT hotspot you connected to was a Homehub so it suggests that the problem is with your particular one. Is it one of the latest models (do a Google search for "telling the difference between Homehub routers" or something similar and compare with yours)? If it is not one of the latest models (i think it is Homehub 3), call BT and see if they will replace it. I have found BT fairly open to negotiation, particularly if you are near the end of your annual contract.