Are you trying to delete those rows, or do you simply want to take away the mailto: part?
Because if you want to delete anything with a mailto, you dont need a trim or susbtr function - you would just do this:
delete from unsub_final4
where email like 'mailto:%'
BUT if you want to just remove the mailto: part and not delete the record, you need to use an update
statement, and either the substr or ltrim (or replace or translate) functions.
Just wanted to clarify, because your delete statement is overkill right there.