diff --git a/app/celery/letters_pdf_tasks.py b/app/celery/letters_pdf_tasks.py
index ec08666b4..7096f281a 100644
--- a/app/celery/letters_pdf_tasks.py
+++ b/app/celery/letters_pdf_tasks.py
@@ -98,7 +98,8 @@ def collate_letter_pdfs_for_day(date):
         notify_celery.send_task(
             name=TaskNames.ZIP_AND_SEND_LETTER_PDFS,
             kwargs={'filenames_to_zip': filenames},
-            queue=QueueNames.PROCESS_FTP
+            queue=QueueNames.PROCESS_FTP,
+            compression='zlib'
         )
 
 
@@ -112,7 +113,10 @@ def group_letters(letter_pdfs):
     list_of_files = []
     for letter in letter_pdfs:
         if letter['Key'].lower().endswith('.pdf'):
-            if running_filesize + letter['Size'] > current_app.config['MAX_LETTER_PDF_ZIP_FILESIZE']:
+            if (
+                running_filesize + letter['Size'] > current_app.config['MAX_LETTER_PDF_ZIP_FILESIZE'] or
+                len(list_of_files) >= current_app.config['MAX_LETTER_PDF_COUNT_PER_ZIP']
+            ):
                 yield list_of_files
                 running_filesize = 0
                 list_of_files = []
diff --git a/app/config.py b/app/config.py
index e42a08c3b..ba24b4a5a 100644
--- a/app/config.py
+++ b/app/config.py
@@ -129,6 +129,7 @@ class Config(object):
     MAX_VERIFY_CODE_COUNT = 10
 
     MAX_LETTER_PDF_ZIP_FILESIZE = 500 * 1024 * 1024  # 500mb
+    MAX_LETTER_PDF_COUNT_PER_ZIP = 5000
 
     CHECK_PROXY_HEADER = False
 
diff --git a/tests/app/celery/test_letters_pdf_tasks.py b/tests/app/celery/test_letters_pdf_tasks.py
index 78a58306c..5c2532934 100644
--- a/tests/app/celery/test_letters_pdf_tasks.py
+++ b/tests/app/celery/test_letters_pdf_tasks.py
@@ -155,16 +155,18 @@ def test_collate_letter_pdfs_for_day(notify_api, mocker):
     assert mock_celery.call_args_list[0] == call(
         name='zip-and-send-letter-pdfs',
         kwargs={'filenames_to_zip': ['A.PDF', 'B.pDf']},
-        queue='process-ftp-tasks'
+        queue='process-ftp-tasks',
+        compression='zlib'
     )
     assert mock_celery.call_args_list[1] == call(
         name='zip-and-send-letter-pdfs',
         kwargs={'filenames_to_zip': ['C.pdf']},
-        queue='process-ftp-tasks'
+        queue='process-ftp-tasks',
+        compression='zlib'
     )
 
 
-def test_group_letters(notify_api):
+def test_group_letters_splits_on_file_size(notify_api):
     letters = [
         # ends under max but next one is too big
         {'Key': 'A.pdf', 'Size': 1}, {'Key': 'B.pdf', 'Size': 2},
@@ -190,6 +192,63 @@ def test_group_letters(notify_api):
         assert next(x, None) is None
 
 
+def test_group_letters_splits_on_file_count(notify_api):
+    letters = [
+        {'Key': 'A.pdf', 'Size': 1},
+        {'Key': 'B.pdf', 'Size': 2},
+        {'Key': 'C.pdf', 'Size': 3},
+        {'Key': 'D.pdf', 'Size': 1},
+        {'Key': 'E.pdf', 'Size': 1},
+        {'Key': 'F.pdf', 'Size': 5},
+        {'Key': 'G.pdf', 'Size': 6},
+        {'Key': 'H.pdf', 'Size': 1},
+        {'Key': 'I.pdf', 'Size': 1},
+    ]
+
+    with set_config_values(notify_api, {'MAX_LETTER_PDF_COUNT_PER_ZIP': 3}):
+        x = group_letters(letters)
+
+        assert next(x) == [{'Key': 'A.pdf', 'Size': 1}, {'Key': 'B.pdf', 'Size': 2}, {'Key': 'C.pdf', 'Size': 3}]
+        assert next(x) == [{'Key': 'D.pdf', 'Size': 1}, {'Key': 'E.pdf', 'Size': 1}, {'Key': 'F.pdf', 'Size': 5}]
+        assert next(x) == [{'Key': 'G.pdf', 'Size': 6}, {'Key': 'H.pdf', 'Size': 1}, {'Key': 'I.pdf', 'Size': 1}]
+        # make sure iterator is exhausted
+        assert next(x, None) is None
+
+
+def test_group_letters_splits_on_file_size_and_file_count(notify_api):
+    letters = [
+        # ends under max file size but next file is too big
+        {'Key': 'A.pdf', 'Size': 1},
+        {'Key': 'B.pdf', 'Size': 2},
+        # ends on exactly max number of files and file size
+        {'Key': 'C.pdf', 'Size': 3},
+        {'Key': 'D.pdf', 'Size': 1},
+        {'Key': 'E.pdf', 'Size': 1},
+        # exactly max file size goes in next file
+        {'Key': 'F.pdf', 'Size': 5},
+        # file size is within max but number of files reaches limit
+        {'Key': 'G.pdf', 'Size': 1},
+        {'Key': 'H.pdf', 'Size': 1},
+        {'Key': 'I.pdf', 'Size': 1},
+        # whatever's left goes in last list
+        {'Key': 'J.pdf', 'Size': 1},
+    ]
+
+    with set_config_values(notify_api, {
+        'MAX_LETTER_PDF_ZIP_FILESIZE': 5,
+        'MAX_LETTER_PDF_COUNT_PER_ZIP': 3
+    }):
+        x = group_letters(letters)
+
+        assert next(x) == [{'Key': 'A.pdf', 'Size': 1}, {'Key': 'B.pdf', 'Size': 2}]
+        assert next(x) == [{'Key': 'C.pdf', 'Size': 3}, {'Key': 'D.pdf', 'Size': 1}, {'Key': 'E.pdf', 'Size': 1}]
+        assert next(x) == [{'Key': 'F.pdf', 'Size': 5}]
+        assert next(x) == [{'Key': 'G.pdf', 'Size': 1}, {'Key': 'H.pdf', 'Size': 1}, {'Key': 'I.pdf', 'Size': 1}]
+        assert next(x) == [{'Key': 'J.pdf', 'Size': 1}]
+        # make sure iterator is exhausted
+        assert next(x, None) is None
+
+
 def test_group_letters_ignores_non_pdfs(notify_api):
     letters = [{'Key': 'A.zip'}]
     assert list(group_letters(letters)) == []