diff --git a/resources/default_tweaks.py b/resources/default_tweaks.py
index cc73dcbd7f..dad8fea0c2 100644
--- a/resources/default_tweaks.py
+++ b/resources/default_tweaks.py
@@ -437,8 +437,11 @@ metadata_edit_custom_column_order = []
 # public email server like gmx/hotmail/gmail. Default is: 5 minutes
 # Setting it to lower may cause the server's SPAM controls to kick in,
 # making email sending fail. Changes will take effect only after a restart of
-# calibre.
+# calibre. You can also change the list of hosts that calibre considers
+# to be public relays here. Any relay host ending with one of the suffixes
+# in the list below will be considered a public email server.
 public_smtp_relay_delay = 301
+public_smtp_relay_host_suffixes = ['gmail.com', 'live.com', 'gmx.com']
 
 #: The maximum width and height for covers saved in the calibre library
 # All covers in the calibre library will be resized, preserving aspect ratio,
diff --git a/src/calibre/gui2/email.py b/src/calibre/gui2/email.py
index d0ef891e9b..cc9e9fd91a 100644
--- a/src/calibre/gui2/email.py
+++ b/src/calibre/gui2/email.py
@@ -62,9 +62,11 @@ class Sendmail(object):
         self.rate_limit = 1
         opts = email_config().parse()
         rh = opts.relay_host
-        if rh and (
-            'gmail.com' in rh or 'live.com' in rh or 'gmx.com' in rh):
-            self.rate_limit = tweaks['public_smtp_relay_delay']
+        if rh:
+            for suffix in tweaks['public_smtp_relay_host_suffixes']:
+                if rh.lower().endswith(suffix):
+                    self.rate_limit = tweaks['public_smtp_relay_delay']
+                    break
 
     def __call__(self, attachment, aname, to, subject, text, log=None,
             abort=None, notifications=None):