diff --git a/sydent/http/servlets/emailservlet.py b/sydent/http/servlets/emailservlet.py
index 077d15f5..1513cdf4 100644
--- a/sydent/http/servlets/emailservlet.py
+++ b/sydent/http/servlets/emailservlet.py
@@ -17,7 +17,7 @@
 
 from twisted.web.resource import Resource
 
-from sydent.util.stringutils import is_valid_client_secret
+from sydent.util.stringutils import is_valid_client_secret, MAX_EMAIL_ADDRESS_LENGTH
 from sydent.util.emailutils import EmailAddressException, EmailSendException
 from sydent.validators import (
     IncorrectClientSecretException,
@@ -58,6 +58,13 @@ def render_POST(self, request):
                 'error': 'Invalid client_secret provided'
             }
 
+        if not (0 < len(email) <= MAX_EMAIL_ADDRESS_LENGTH):
+            request.setResponseCode(400)
+            return {
+                'errcode': 'M_INVALID_PARAM',
+                'error': 'Invalid email provided'
+            }
+
         ipaddress = self.sydent.ip_from_request(request)
         brand = self.sydent.brand_from_request(request)
 
diff --git a/sydent/http/servlets/store_invite_servlet.py b/sydent/http/servlets/store_invite_servlet.py
index 3dce2a11..e30b9c19 100644
--- a/sydent/http/servlets/store_invite_servlet.py
+++ b/sydent/http/servlets/store_invite_servlet.py
@@ -30,6 +30,8 @@
 from sydent.http.servlets import get_args, send_cors, jsonwrap, MatrixRestError
 from sydent.http.auth import authV2
 from sydent.util.emailutils import sendEmail
+from sydent.util.stringutils import MAX_EMAIL_ADDRESS_LENGTH
+
 
 class StoreInviteServlet(Resource):
     def __init__(self, syd, require_auth=False):
@@ -71,6 +73,13 @@ def render_POST(self, request):
                 "error": "Didn't understand medium '%s'" % (medium,),
             }
 
+        if not (0 < len(address) <= MAX_EMAIL_ADDRESS_LENGTH):
+            request.setResponseCode(400)
+            return {
+                'errcode': 'M_INVALID_PARAM',
+                'error': 'Invalid email provided'
+            }
+
         token = self._randomString(128)
 
         tokenStore = JoinTokenStore(self.sydent)
diff --git a/sydent/util/stringutils.py b/sydent/util/stringutils.py
index fdf24009..b4726403 100644
--- a/sydent/util/stringutils.py
+++ b/sydent/util/stringutils.py
@@ -18,7 +18,7 @@
 from twisted.internet.abstract import isIPAddress, isIPv6Address
 
 # https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-register-email-requesttoken
-client_secret_regex = re.compile(r"^[0-9a-zA-Z\.\=\_\-]+$")
+CLIENT_SECRET_REGEX = re.compile(r"^[0-9a-zA-Z\.=_\-]+$")
 
 # hostname/domain name
 # https://regex101.com/r/OyN1lg/2
@@ -26,6 +26,15 @@
     r"^(?:[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?)(?:\.[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?)*$",
     flags=re.IGNORECASE)
 
+# it's unclear what the maximum length of an email address is. RFC3696 (as corrected
+# by errata) says:
+#    the upper limit on address lengths should normally be considered to be 254.
+#
+# In practice, mail servers appear to be more tolerant and allow 400 characters
+# or so. Let's allow 500, which should be plenty for everyone.
+#
+MAX_EMAIL_ADDRESS_LENGTH = 500
+
 
 def is_valid_client_secret(client_secret):
     """Validate that a given string matches the client_secret regex defined by the spec
@@ -36,7 +45,10 @@ def is_valid_client_secret(client_secret):
     :return: Whether the client_secret is valid
     :rtype: bool
     """
-    return client_secret_regex.match(client_secret) is not None
+    return (
+        0 < len(client_secret) <= 255
+        and CLIENT_SECRET_REGEX.match(client_secret) is not None
+    )
 
 
 def is_valid_hostname(string: str) -> bool: