Hi Jason - just a quick follow up on this. It turns out that the current hosting does continue after blank or invalid emails, so we had a bit of a bum steer about that from them. They have now just advised us that there is a script timeout, so we need to break the mail shot out into batches. I assume this then resets the script timeout when each batch starts.
Looking at the UE page here:
Under the section on how to avoid getting blocked by hosting providers, it mentions sending batch emails. From the screen shot it's not obvious how to do that though - is that possible? Is it just the figure in the Wait box for example? Ie that figure is the gap between batches, rather than between emails?