feat: Provide users possible status codes that SendGrid can return

src/SendGrid/Helpers/Errors/ErrorHandler.cs

@@ -87,7 +87,7 @@ public static async Task ThrowException(HttpResponseMessage message)
         /// <returns>Return string with the error Status Code and the Message</returns>
         private static async Task<string> GetErrorMessage(HttpResponseMessage message)
         {
-            var errorStatusCode = (int)message.StatusCode;
+            var errorStatusCode = (SendGridStatusCode)message.StatusCode;
             var errorReasonPhrase = message.ReasonPhrase;
 
             string errorValue = null;

src/SendGrid/Helpers/Errors/Model/SendGridErrorResponse.cs

@@ -6,9 +6,10 @@
     public class SendGridErrorResponse
     {
         /// <summary>
-        /// Gets or sets the error Status Code
+        /// Gets or sets the error Status Code.<br/>
+        /// Might be any <see cref="System.Net.HttpStatusCode"/>, not just ones in <see cref="SendGridStatusCode"/>.
         /// </summary>
-        public int ErrorHttpStatusCode { get; set; }
+        public SendGridStatusCode ErrorHttpStatusCode { get; set; }
 
         /// <summary>
         /// Gets or sets the error Reason Phrase

src/SendGrid/Helpers/Errors/Model/SendGridStatusCode.cs

@@ -0,0 +1,105 @@
+namespace SendGrid.Helpers.Errors.Model
+{
+    /// <summary>
+    /// Each SMTP call you make returns a response.
+    /// 200 responses are usually success responses, and 400 responses are usually deferrals.
+    /// SendGrid continues to retry resending 400 messages for up to 72 hours.
+    /// 500 responses are hard failures that are not retried by our servers.
+    /// </summary>
+    /// <remarks>
+    /// <see href="https://docs.sendgrid.com/for-developers/sending-email/smtp-errors-and-troubleshooting"/>.
+    /// </remarks>
+    public enum SendGridStatusCode
+    {
+        /// <summary>
+        /// Your mail has been successfully queued!
+        /// This response indicates that the recipient server has accepted the message.
+        /// </summary>
+        QueuedForDelivery = 250,
+
+        /// <summary>
+        /// This means the "from" address does not match a verified Sender Identity.
+        /// Mail cannot be sent until this error is resolved.
+        /// </summary>
+        /// <remarks>
+        /// To learn how to resolve this error, see our <see href="https://docs.sendgrid.com/for-developers/sending-email/sender-identity">Sender Identity requirements</see>.
+        /// </remarks>
+        InvalidFromAddress = 403,
+
+        /// <summary>
+        /// Messages are temporarily deferred because of recipient server policy - often it's because of too many messages or connections in too short of a timeframe.
+        /// </summary>
+        /// <remarks>
+        /// We continue to retry deferred messages for up to 72 hours.
+        /// Consider temporarily sending less messages to a domain that is returning this code because this could further delay your messages currently being tried.
+        /// </remarks>
+        MessagesDeferred = 421,
+
+        /// <summary>
+        /// The message failed because the recipient's mailbox was unavailable, perhaps because it was locked or was not routable at the time.
+        /// </summary>
+        /// <remarks>
+        /// We continue to retry messages for up to 72 hours.
+        /// Consider temporarily sending less messages to a domain that is returning this code because this could further delay your messages currently being tried.
+        /// </remarks>
+        MailboxUnavailable = 450,
+
+        /// <summary>
+        /// There is a credit limit of emails per day enforced in error.
+        /// </summary>
+        /// <remarks>
+        /// <see href="https://support.sendgrid.com/hc">Contact support</see> to remove that limit.
+        /// </remarks>
+        MaximumCreditsExceeded = 451,
+
+        /// <summary>
+        /// The message has been deferred due to insufficient system storage.
+        /// </summary>
+        /// <remarks>
+        /// We continue to retry messages for up to 72 hours.
+        /// </remarks>
+        TooManyRecipientsThisHour = 452,
+
+        /// <summary>
+        /// The user’s mailbox was unavailable.
+        /// Usually because it could not be found, or because of incoming policy reasons.
+        /// </summary>
+        /// <remarks>
+        /// Remove these address from your list - it is likely a fake, or it was mistyped.
+        /// </remarks>
+        InvalidMailbox = 550,
+
+        /// <summary>
+        /// The intended mailbox does not exist on this recipient server.
+        /// </summary>
+        /// <remarks>
+        /// Remove these addresses from your list.
+        /// </remarks>
+        UserDoesNotExist = 551,
+
+        /// <summary>
+        /// The recipients mailbox has exceeded its storage limits.
+        /// </summary>
+        /// <remarks>
+        /// We don't resend messages with this error code because this is usually a sign this is an abandoned email.
+        /// </remarks>
+        MailboxIsFull = 552,
+
+        /// <summary>
+        /// The message was refused because the mailbox name is either malformed or does not exist.
+        /// </summary>
+        /// <remarks>
+        /// Remove these addresses from your list.
+        /// </remarks>
+        InvalidUser = 553,
+
+        /// <summary>
+        /// This is a default response that can be caused by a lot of issues.
+        /// </summary>
+        /// <remarks>
+        /// There is often a human readable portion of this error that gives more detailed information, but if not,
+        /// remove these addresses from your list.
+        /// </remarks>
+        MailRefused = 554,
+    }
+}

tests/SendGrid.Tests/Integration.cs

@@ -6148,7 +6148,7 @@ public async void TestHttpErrorAsException()
 
                 var errorResponseExpected = new SendGridErrorResponse
                 {
-                    ErrorHttpStatusCode = 503,
+                    ErrorHttpStatusCode = (SendGridStatusCode)503,
                     ErrorReasonPhrase = "Service Unavailable",
                     SendGridErrorMessage = "error message",
                     FieldWithError = "field value",