I had this when I was trying to send via a non NTL SMTP server...
change the account to send via the NTL smtp, and all should work nicely. (at least it did for me
)
(I made the assumption that NTL had updated their SMTP server to stop relaying)
the SMTP server does not care what "from" address you use, but will not allow you to go to another SMTP server directly.
Hope this helps...
<edit> - bugger must read the rest of the posts before replying -
I can see no reason for this error, unless the outlook accounts are trying to use a differnt SMTP server - as they do not appear to do so, this could be a timeout issue.
Is there a pattern to this, or is it truly random...