Neither you or Thunderbird can control if the recipient has forwarded their email address to a dead address. Thunderbird did its task buy handing the message to the server the original address resides on. I suggest you contact the recipient and tell them they need to remove the forwarding.

RPrideaux said

I am trying to send an email to one of my previous colleagues. His work email address has now been deleted so I am sending to his personal address. However, at some point in the past the two email addresses have been linked so now whenever I send to his ntlworld.com (personal) address, is it redirected to his nottinghamarimathea.org.uk (work) address. As this address no longer exists, delivery fails and he receives a failure notice to his ntlworld.com address. This only happens when sending via Thunderbird. If I send emails via webmail.nottinghamarimathea.org.uk there is no problems at all. Any advice would be welcome! Thanks

The problem is that this only happens when emails are send from Thunderbird - so it's not that he has redirected his emails to a dead address because emails sent though other platforms are going through fine.

Thunderbird would have no knowledge of a second address to send it to unless you or your address book provides it. Thunderbird sends it to the address you place in the address field. It is that simple. Do you have a second entry in one of your address book with the old address and are using the wrong one?