What you are showing there does not appear to be a issue with the IronMail, as the log states that the other server disconnected. It would probably be best to get a packet capture to show what is truly going on. You can run this at the command line, but that is hard to read. The easiest step may to be to call support and have them run the packet capture while the message is retried. This should be fairly straight forward, as the next hop mail server is not supporting TLS, so any errors would be in plain text.