Hi Andrew,
With outgoing email it tends to be more likely to be the recipient's spam filtering that is blocking your emails.
I would advise contacting the recipients via another email address and ask them to check with their email providers if there are any blocks on BT hosted addresses. If there aren't then you should probably contact the Helpdesk.
Additional: I just re-read your post and you were able to get through with an @btconnect.com address. That indicates that a block on BT is less likely, but your domain may be blacklisted. Check on some of the blacklisting sites, for example MX Toolbox.
Dave
