Anyone know why after running a script which sends out an email the email shows in the sent Email list but has status not sent?

DKIM set up and properly tested in Email Preferences?

its in SB and already using this function on several other scripts.

Check email preference

Test email address not added in Email preferences > Sandbox and Release Preview after last refresh?

all works, am admin and get the error message for instance when the script fails

Are you sending multiple emails with similar/same subject? I seem to recall there's something which stops apparent duplicates.

no, just testing on 1

I'd try adding a unique value in the subject just to be sure. We've had issues in the past with successive confirmation emails to customers suddenly stopping. Adding the order ID into the subject solved it. There are two different email send methods within Netsuite - regular and bulk. If the server sees multiple identical mails sent via standard, it blocks them as potential junk.