diff --git a/paypal-adaptive-hoops.cabal b/paypal-adaptive-hoops.cabal
--- a/paypal-adaptive-hoops.cabal
+++ b/paypal-adaptive-hoops.cabal
@@ -1,5 +1,5 @@
 name:                 paypal-adaptive-hoops
-version:              0.5.3.0
+version:              0.6.0.0
 author:               Ian Grant Jeffries
 maintainer:           ian@housejeffries.com
 category:             Web
@@ -20,6 +20,7 @@
   default-language:   Haskell2010
   ghc-options:        -Wall
   other-extensions:   GADTs
+                    , MultiWayIf
                     , OverloadedStrings
                     , TemplateHaskell
 
diff --git a/src/Web/PayPal/Adaptive.hs b/src/Web/PayPal/Adaptive.hs
--- a/src/Web/PayPal/Adaptive.hs
+++ b/src/Web/PayPal/Adaptive.hs
@@ -6,6 +6,7 @@
   , ReceiverList     (..)
   , SendPayment      (..)
   , CreatePayment    (..)
+  , approvalUrl
   , LookupPayment    (..)
   , PayResp          (..)
   , PayExecStatus    (..)
@@ -15,7 +16,6 @@
   , TransactionId    (..)
   , TransactionStatus(..)
   , Money            (..)
-  , approvalUrl
   , m2Currency
   , m2PayPal
   ) where
diff --git a/src/Web/PayPal/Adaptive/Core.hs b/src/Web/PayPal/Adaptive/Core.hs
--- a/src/Web/PayPal/Adaptive/Core.hs
+++ b/src/Web/PayPal/Adaptive/Core.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE GADTs             #-}
+{-# LANGUAGE MultiWayIf        #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TemplateHaskell   #-}
 
@@ -19,8 +20,8 @@
 import           Data.Monoid
 import           Data.Text            (Text)
 import qualified Data.Text            as T
-import           Data.Text.Read
 import           Data.Text.Encoding
+import           Data.Text.Read
 import           Data.Traversable
 import qualified Data.Vector          as V
 import           Network.HTTP.Client  (HttpException (NoResponseDataReceived))
@@ -80,10 +81,17 @@
   in case eitherDecode b of
     Right d -> Right d
     Left  e -> -- If the response isn't a PayResp, it might be a PayPal error message.
-      case eitherDecode b of
-        Right (AeErrCodes cs) -> Left $ AeErrCodes cs
-        _                     -> Left $ AeDecodeFailed b (T.pack e)
+      case decode b of
+        Just (AeErrCodes codes) -> Left $ codeErr codes
+        _ -> Left $ AeDecodeFailed b (T.pack e)
 
+  where
+    codeErr :: [Int] -> AdaptiveErr
+    codeErr cs =
+      if | elem 520003 cs -> AeInvalidCredentials
+         | elem 589039 cs -> AeNoSuchEmail
+         | otherwise      -> AeErrCodes cs
+
 --------------------------------------------------
 -- Errors
 --------------------------------------------------
@@ -93,10 +101,15 @@
 -- AeShouldNotHappen is meant to cover PayPal responses to API requests
 -- that this library doesn't support sending in the first place.
 -- TODO: is this actually how we're using it?
+--
+-- AeNoSuchEmail is raised when the CreatePayment sender
+-- email doesn't exist.
 data AdaptiveErr
   = AeNoResponse
   | AeConnectionErr   Text
   | AeDecodeFailed    ByteString Text
+  | AeInvalidCredentials
+  | AeNoSuchEmail
   | AeErrCodes        [Int]
   | AePending         PayResp
   | AeRefused         PayResp
@@ -199,13 +212,15 @@
             let infos = _prPayInfo a
             in if length infos /= 1
               then Left $ AeShouldNotHappen a "sendPayment expects one PayInfo in reponse"
-              else case _piTransactionStatus (head infos) of
+              else
+                let info = head infos
+                in case _piTransactionStatus info of
                 Just TsCompleted  -> Right a
                 Just TsPending    -> Left $ AePending a
                 Just TsProcessing -> Left $ AePending a
                 Just TsDenied     -> Left $ AeRefused a
                 Just TsFailed     -> Left $ AeRefused a
-                _            -> Left $ AeShouldNotHappen a "got unsupported TransactionStatus"
+                _ -> Left $ AeShouldNotHappen a "got unsupported TransactionStatus"
           _ -> Left $ AeShouldNotHappen a "got unsupported PayExecStatus"
 
 --------------------------------------------------