Envelope enhancements

Build.ps1

@@ -10,7 +10,7 @@ if(Test-Path .\artifacts) {
 }
 
 & dotnet restore --no-cache
-if($LASTEXITCODE -ne 0) { throw "Build failed with exit code $LASTEXITCODE" }    
+if($LASTEXITCODE -ne 0) { exit 1 }    
 
 $branch = @{ $true = $env:APPVEYOR_REPO_BRANCH; $false = $(git symbolic-ref --short -q HEAD) }[$env:APPVEYOR_REPO_BRANCH -ne $NULL];
 $revision = @{ $true = "{0:00000}" -f [convert]::ToInt32("0" + $env:APPVEYOR_BUILD_NUMBER, 10); $false = "local" }[$env:APPVEYOR_BUILD_NUMBER -ne $NULL];
@@ -30,7 +30,7 @@ foreach ($src in ls src/*) {
         & dotnet publish -c Release -o ./obj/publish
         & dotnet pack -c Release -o ..\..\artifacts --no-build
     }
-    if($LASTEXITCODE -ne 0) { throw "Build failed with exit code $LASTEXITCODE" }    
+    if($LASTEXITCODE -ne 0) { exit 1 }    
 
     Pop-Location
 }
@@ -41,9 +41,9 @@ foreach ($test in ls test/*.Tests) {
     echo "build: Testing project in $test"
 
     & dotnet test -c Release
-    if($LASTEXITCODE -ne 0) { throw "Build failed with exit code $LASTEXITCODE" }
+    if($LASTEXITCODE -ne 0) { exit 3 }
 
     Pop-Location
 }
 
-Pop-Location
+Pop-Location

appveyor.yml

@@ -1,6 +1,6 @@
 version: '{build}'
 skip_tags: true
-image: Visual Studio 2022
+image: Visual Studio 2019
 install:
 build_script:
   - pwsh: ./Build.ps1

src/Seq.App.EmailPlus/DeliveryType.cs

@@ -0,0 +1,12 @@
+namespace Seq.App.EmailPlus
+{
+    public enum DeliveryType
+    {
+        MailHost,
+        MailFallback,
+        Dns,
+        DnsFallback,
+        HostDnsFallback,
+        None = -1
+    }
+}

src/Seq.App.EmailPlus/DirectMailGateway.cs

@@ -1,23 +1,203 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
+using DnsClient;
+using DnsClient.Protocol;
 using MailKit.Net.Smtp;
+using MailKit.Security;
 using MimeKit;
 
 namespace Seq.App.EmailPlus
 {
     class DirectMailGateway : IMailGateway
     {
-        public async Task SendAsync(SmtpOptions options, MimeMessage message)
+        static readonly SmtpClient Client = new SmtpClient();
+        static readonly LookupClient DnsClient = new LookupClient();
+
+        public async Task<MailResult> SendAsync(SmtpOptions options, MimeMessage message)
+        {
+            if (message == null) throw new ArgumentNullException(nameof(message));
+            var mailResult = new MailResult();
+            var type = DeliveryType.MailHost;
+            var errors = new List<Exception>();
+            foreach (var server in options.Host)
+            {
+                mailResult = await TryDeliver(server, options, message, type);
+                if (!mailResult.Success)
+                {
+                    errors.Add(mailResult.LastError);
+                    type = DeliveryType.MailFallback;
+                }
+                else
+                {
+                    break;
+                }
+            }
+
+            mailResult.Errors = errors;
+            return mailResult;
+        }
+
+        public async Task<DnsMailResult> SendDnsAsync(DeliveryType deliveryType, SmtpOptions options,
+            MimeMessage message)
         {
+            var dnsResult = new DnsMailResult();
+            var resultList = new List<MailResult>();
+            var lastServer = string.Empty;
+            var errors = new List<Exception>();
             if (message == null) throw new ArgumentNullException(nameof(message));
+            var type = deliveryType;
+
+            try
+            {
+                var domains = GetDomains(message).ToList();
+                var successCount = 0;
+
+                foreach (var domain in domains)
+                {
+                    type = deliveryType;
+                    lastServer = domain;
+                    var mx = await DnsClient.QueryAsync(domain, QueryType.MX);
+                    var mxServers =
+                        (from mxServer in mx.Answers
+                            where !string.IsNullOrEmpty(((MxRecord)mxServer).Exchange)
+                            select ((MxRecord)mxServer).Exchange).Select(dummy => (string)dummy).ToList();
+                    var mailResult = new MailResult();
+                    foreach (var server in mxServers)
+                    {
+                        lastServer = server;
+                        mailResult = await TryDeliver(server, options, message, type);
+                        var lastError = dnsResult.LastError;
+                        errors.AddRange(mailResult.Errors);
+
+                        dnsResult = new DnsMailResult
+                        {
+                            LastServer = server,
+                            LastError = mailResult.LastError ?? lastError,
+                            Type = type,
+                        };
+
+                        resultList.Add(mailResult);
+
+                        if (mailResult.Success)
+                            break;
+                        type = DeliveryType.DnsFallback;
+                    }
+
+                    if (mailResult.Success)
+                    {
+                        successCount++;
+                        continue;
+                    }
+
+                    if (dnsResult.LastError != null) continue;
+                    dnsResult.LastError = mxServers.Count == 0
+                        ? new Exception("DNS delivery failed - no MX records detected for " + domain)
+                        : new Exception("DNS delivery failed - no error detected");
+                    dnsResult.Success = false;
+                    dnsResult.Errors.Add(dnsResult.LastError);
+                }
+
+                if (!domains.Any())
+                {
+                    dnsResult.Success = false;
+                    dnsResult.LastError =
+                        new Exception("DNS delivery failed - no domains parsed from recipient addresses");
+                }
+
+                if (successCount < domains.Count)
+                {
+                    if (successCount == 0)
+                    {
+                        dnsResult.Success = false;
+                        dnsResult.LastError =
+                            new Exception("DNS delivery failure - no domains could be successfully delivered.");
+                    }
+                    else
+                    {
+                        dnsResult.Success = true; // A qualified success ...
+                        dnsResult.LastError =
+                            new Exception(
+                                $"DNS delivery partial failure - {domains.Count - successCount} of {successCount} domains could not be delivered.");
+                    }
+
+                    dnsResult.Errors.Add(dnsResult.LastError);
+                }
+                else
+                {
+                    dnsResult.Success = true;
+                }
+            }
+            catch (Exception ex)
+            {
+                dnsResult = new DnsMailResult
+                {
+                    Type = type,
+                    LastServer = lastServer,
+                    Success = false,
+                    LastError = ex
+                };
+            }
+
+            dnsResult.Errors = errors;
+            dnsResult.Results = resultList;
+            return dnsResult;
+        }
+
+
+        public static IEnumerable<string> GetDomains(MimeMessage message)
+        {
+            var domains = new List<string>();
+            foreach (var to in message.To)
+            {
+                var toDomain = to.ToString().Split('@')[1];
+                if (string.IsNullOrEmpty(toDomain)) continue;
+                if (!domains.Any(domain => domain.Equals(toDomain, StringComparison.OrdinalIgnoreCase)))
+                    domains.Add(toDomain);
+            }
+
+            foreach (var cc in message.Cc)
+            {
+                var ccDomain = cc.ToString().Split('@')[1];
+                if (string.IsNullOrEmpty(ccDomain)) continue;
+                if (!domains.Any(domain => domain.Equals(ccDomain, StringComparison.OrdinalIgnoreCase)))
+                    domains.Add(ccDomain);
+            }
+
+
+            foreach (var bcc in message.Bcc)
+            {
+                var bccDomain = bcc.ToString().Split('@')[1];
+                if (string.IsNullOrEmpty(bccDomain)) continue;
+                if (!domains.Any(domain => domain.Equals(bccDomain, StringComparison.OrdinalIgnoreCase)))
+                    domains.Add(bccDomain);
+            }
+
+            return domains;
+        }
+
+        private static async Task<MailResult> TryDeliver(string server, SmtpOptions options, MimeMessage message,
+            DeliveryType deliveryType)
+        {
+            if (string.IsNullOrEmpty(server))
+                return new MailResult {Success = false, LastServer = server, Type = deliveryType};
+            try
+            {
+                await Client.ConnectAsync(server, options.Port, (SecureSocketOptions)options.SocketOptions);
+                if (options.RequiresAuthentication)
+                    await Client.AuthenticateAsync(options.Username, options.Password);
+
+
+                await Client.SendAsync(message);
+                await Client.DisconnectAsync(true);
 
-            var client = new SmtpClient();
-
-            await client.ConnectAsync(options.Host, options.Port, options.SocketOptions);
-            if (options.RequiresAuthentication)
-                await client.AuthenticateAsync(options.Username, options.Password);
-            await client.SendAsync(message);
-            await client.DisconnectAsync(true);
+                return new MailResult {Success = true, LastServer = server, Type = deliveryType};
+            }
+            catch (Exception ex)
+            {
+                return new MailResult {Success = false, LastError = ex, LastServer = server, Type = deliveryType};
+            }
         }
     }
 }

src/Seq.App.EmailPlus/DnsMailResult.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+// ReSharper disable UnusedAutoPropertyAccessor.Global
+
+namespace Seq.App.EmailPlus
+{
+    public class DnsMailResult
+    {
+        public bool Success { get; set; }
+        public DeliveryType Type { get; set; }
+        public string LastServer { get; set; }
+        public Exception LastError { get; set; }
+        public List<Exception> Errors { get; set; } = new List<Exception>();
+        public List<MailResult> Results { get; set; } = new List<MailResult>();
+    }
+}