<example@example.co.uk>:
Connected to x.x.x.x but connection died. (#4.4.2) I'm not going to try again; this message has been in the queue too long.
Because you have obfuscated the error message, nobody else can investigate further.
There is either a DNS configuration error for the target domain, a problem in one or more of their mail servers, or a connectivity problem between your site and port 25 of one or more of their mail servers. None of those issues is an SME server issue.