In order to see where things are going wrong, hontreasurer.com have added a trace to the email system. After sending an email, a file called hontreasurer.com_Trace_Output.txt is written to your system. This will be in different places depending on whether you are using Windows (normally in My Documents), linux or Mac so you will probably have to search for it. Look at the last few lines. There should be some clue as to which address caused it to stop.