The QV hangs up due to the fact that for some reason the notification refuses to be sent to the standard mailing address. This does not happen often, but greatly affects the work. Is there any way around this problem so qlik does not hang?
In regard to invalid mail-addresses you need to clean and/or to skip them - means to check the length, the exists of the char @, removing all invalid chars and so on.
If you could ensure that's not cause and it happens occasionally at night you may hit a maintaining-window or any overload on your mail-server, storage/network or similar. I don't know if the inbuilt alert-features is able to skip any error and/or if you could chain any pre-checks to your real-task (I assume rather not). By using vbs-macros you are in general able to implement an error-handling but I think it will also have some limitations and probably not very easy to implement.