Complex sentence: For a mailing company, data entry clerks might be required to type in reference numbers for items of mail which had failed to reach their destination, so that the relevant addresses could be deleted from the database used to send the mail out.
Split into simpler sentences: For a mailing company, data entry clerks might be required to type in reference numbers for items of mail, which had failed to reach their destination. So that the relevant addresses could be deleted from the database used to send the mail out.