This can happen for two different reasons:

• Someone who is not on the list has tried sending to the list
• Someone on the list is entered with their iSolutions username e.g. wx3v05@soton.ac.uk instead of their mailname e.g. w.xyz@soton.ac.uk

When compiling the list, you must consider who is going to send to the list. It might be necessary to ask the user for their mailname, which is usually in the format initial.surname@soton.ac.uk

If a user will only be a recipient for messages, you can simply use the format username@soton.ac.uk