Docs: add info on handling transient ESP errors

Closes #55

docs/sending/exceptions.rst

@@ -35,6 +35,11 @@ Exceptions
     help explain what went wrong. (Tip: you may also be able to check the API log in
     your ESP's dashboard. See :ref:`troubleshooting`.)
 
+    In production, it's not unusual for sends to occasionally fail due to transient
+    connectivity problems, ESP maintenance, or other operational issues. Typically
+    these failures have a 5xx :attr:`status_code`. See :ref:`transient-errors`
+    for suggestions on retrying these failed sends.
+
 
 .. exception:: AnymailInvalidAddress
 

docs/tips/index.rst

@@ -7,6 +7,7 @@ done with Anymail:
 .. toctree::
    :maxdepth: 1
 
+   transient_errors
    multiple_backends
    django_templates
    securing_webhooks

docs/tips/transient_errors.rst

@@ -0,0 +1,25 @@
+.. _transient-errors:
+
+Handling transient errors
+=========================
+
+Applications using Anymail need to be prepared to deal with connectivity issues
+and other transient errors from your ESP's API (as with any networked API).
+
+Because Django doesn't have a built-in way to say "try this again in a few moments,"
+Anymail doesn't have its own logic to retry network errors. The best way to handle
+transient ESP errors depends on your Django project:
+
+* If you already use something like :pypi:`celery` or :pypi:`Django channels <channels>`
+  for background task scheduling, that's usually the best choice for handling Anymail sends.
+  Queue a task for every send, and wait to mark the task complete until the send succeeds
+  (or repeatedly fails, according to whatever logic makes sense for your app).
+
+* Another option is the Pinax :pypi:`django-mailer` package, which queues and automatically
+  retries failed sends for any Django EmailBackend, including Anymail. django-mailer maintains
+  its send queue in your regular Django DB, which is a simple way to get started but may not
+  scale well for very large volumes of outbound email.
+
+In addition to handling connectivity issues, either of these approaches also has the advantage
+of moving email sending to a background thread. This is a best practice for sending email from
+Django, as it allows your web views to respond faster.