diff --git a/app/celery/letters_pdf_tasks.py b/app/celery/letters_pdf_tasks.py
index 5f715c97d..75c2e5c62 100644
--- a/app/celery/letters_pdf_tasks.py
+++ b/app/celery/letters_pdf_tasks.py
@@ -182,15 +182,15 @@ def get_key_and_size_of_letters_to_be_sent_to_print(print_run_deadline, postage)
         try:
             letter_file_name = get_letter_pdf_filename(
                 reference=letter.reference,
-                crown=letter.service.crown,
+                crown=letter.crown,
                 created_at=letter.created_at,
-                postage=letter.postage
+                postage=postage
             )
             letter_head = s3.head_s3_object(current_app.config['LETTERS_PDF_BUCKET_NAME'], letter_file_name)
             yield {
                 "Key": letter_file_name,
                 "Size": letter_head['ContentLength'],
-                "ServiceId": str(letter.service.id)
+                "ServiceId": str(letter.service_id)
             }
         except BotoClientError as e:
             current_app.logger.exception(
diff --git a/app/dao/notifications_dao.py b/app/dao/notifications_dao.py
index 7c8564489..aa57779f7 100644
--- a/app/dao/notifications_dao.py
+++ b/app/dao/notifications_dao.py
@@ -717,11 +717,30 @@ def notifications_not_yet_sent(should_be_sending_after_seconds, notification_typ
     return notifications
 
 
-def dao_get_letters_to_be_printed(print_run_deadline, postage):
+def dao_get_letters_to_be_printed(print_run_deadline, postage, query_limit=10000):
     """
-    Return all letters created before the print run deadline that have not yet been sent
+    Return all letters created before the print run deadline that have not yet been sent. This yields in batches of 10k
+    to prevent the query taking too long and eating up too much memory. As each 10k batch is yielded, the
+    get_key_and_size_of_letters_to_be_sent_to_print function will go and fetch the s3 data, andhese  start sending off
+    tasks to the notify-ftp app to send them.
+
+    CAUTION! Modify this query with caution. Modifying filters etc is fine, but if we join onto another table, then
+    there may be undefined behaviour. Essentially we need each ORM object returned for each row to be unique,
+    and we should avoid modifying state of returned objects.
+
+    For more reading:
+    https://docs.sqlalchemy.org/en/13/orm/query.html?highlight=yield_per#sqlalchemy.orm.query.Query.yield_per
+    https://www.mail-archive.com/sqlalchemy@googlegroups.com/msg12443.html
     """
-    notifications = Notification.query.filter(
+    notifications = db.session.query(
+        Notification.id,
+        Notification.created_at,
+        Notification.reference,
+        Notification.service_id,
+        Service.crown,
+    ).join(
+        Notification.service
+    ).filter(
         Notification.created_at < convert_bst_to_utc(print_run_deadline),
         Notification.notification_type == LETTER_TYPE,
         Notification.status == NOTIFICATION_CREATED,
@@ -730,7 +749,7 @@ def dao_get_letters_to_be_printed(print_run_deadline, postage):
     ).order_by(
         Notification.service_id,
         Notification.created_at
-    ).limit(50000)
+    ).yield_per(query_limit)
     return notifications
 
 
diff --git a/tests/app/dao/notification_dao/test_notification_dao.py b/tests/app/dao/notification_dao/test_notification_dao.py
index 8b8717740..b0e4938b2 100644
--- a/tests/app/dao/notification_dao/test_notification_dao.py
+++ b/tests/app/dao/notification_dao/test_notification_dao.py
@@ -1688,10 +1688,20 @@ def test_letters_to_be_printed_sort_by_service(notify_db_session):
     second_service = create_service(service_name='second service', service_id='642bf33b-54b5-45f2-8c13-942a46616704')
     first_template = create_template(service=first_service, template_type='letter', postage='second')
     second_template = create_template(service=second_service, template_type='letter', postage='second')
-    notification_1 = create_notification(template=first_template, created_at=datetime(2020, 12, 1, 9, 30))
-    notification_2 = create_notification(template=first_template, created_at=datetime(2020, 12, 1, 12, 30))
-    notification_3 = create_notification(template=second_template, created_at=datetime(2020, 12, 1, 8, 30))
+    letters_ordered_by_service_then_time = [
+        create_notification(template=first_template, created_at=datetime(2020, 12, 1, 9, 30)),
+        create_notification(template=first_template, created_at=datetime(2020, 12, 1, 12, 30)),
+        create_notification(template=first_template, created_at=datetime(2020, 12, 1, 13, 30)),
+        create_notification(template=first_template, created_at=datetime(2020, 12, 1, 14, 30)),
+        create_notification(template=first_template, created_at=datetime(2020, 12, 1, 15, 30)),
+        create_notification(template=second_template, created_at=datetime(2020, 12, 1, 8, 30)),
+        create_notification(template=second_template, created_at=datetime(2020, 12, 1, 8, 31)),
+        create_notification(template=second_template, created_at=datetime(2020, 12, 1, 8, 32)),
+        create_notification(template=second_template, created_at=datetime(2020, 12, 1, 8, 33)),
+        create_notification(template=second_template, created_at=datetime(2020, 12, 1, 8, 34))
+    ]
 
-    results = list(dao_get_letters_to_be_printed(print_run_deadline=datetime(2020, 12, 1, 17, 30), postage='second'))
-    assert len(results) == 3
-    assert results == [notification_1, notification_2, notification_3]
+    results = list(
+        dao_get_letters_to_be_printed(print_run_deadline=datetime(2020, 12, 1, 17, 30), postage='second', query_limit=4)
+    )
+    assert [x.id for x in results] == [x.id for x in letters_ordered_by_service_then_time]