Sender limits (#14)

* Add sender limits to missing email

* Formatting

* Add mandrill to sender limits

help/missing_mail.rst

@@ -58,6 +58,27 @@ causes:
 * The sending domain, IP address, or receiving address has been `blacklisted
   <https://mailsac.com/docs/api/#check-blacklist>`_
 
+Sender Limits
+-------------
+Some services place restrictions on the size, type, rate, and quantity of
+email that can be sent.
+
+GMail
+^^^^^
+GMail is not designed for sending bulk mail. The service places `limits 
+<https://support.google.com/mail/answer/22839?hl=en>`_ on the number of
+emails sent per day and the number of recipients.
+
+SendGrid
+^^^^^^^^
+Messages sent through SendGrid are often queued and may not be sent 
+immediately.
+
+Mandrill
+^^^^^^^^
+Messages sent through Mandrall are often `queued <https://mandrill.zendesk.com/hc/en-us/articles/205582717-Why-does-a-delivered-message-say-queued->`_
+and are not sent immediately.
+
 Contacting Support
 ------------------