[WIP] Fix handling of emails with missing headers

Korga.sln

@@ -15,7 +15,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "_", "_", "{28945597-DB66-4C
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "docs", "docs", "{2ED6D7FB-8B8A-4139-98C3-854E00A38490}"
 	ProjectSection(SolutionItems) = preProject
-		docs\docker-compose.yml = docs\docker-compose.yml
+		docs\compose.yaml = docs\compose.yaml
 		docs\email-processing-pipeline.md = docs\email-processing-pipeline.md
 		docs\frontend.md = docs\frontend.md
 	EndProjectSection

server/Korga/EmailRelay/EmailRelayJobController.cs

@@ -38,12 +38,22 @@ public EmailRelayJobController(DatabaseContext database, ILogger<EmailRelayJobCo
 
     protected override async ValueTask ExecuteJob(InboxEmail email, CancellationToken cancellationToken)
     {
+        if (email.From == null)
+        {
+            await RejectEmail(email, errorMessage: null, cancellationToken);
+
+            logger.LogWarning("Email #{Id} has no From header and will be rejected. " +
+                "According to RFC 5311 section 3.6, emails must include a From header. " +
+                "Please contact your email service provider.", email.Id);
+            return;
+        }
+
         if (email.Receiver == null)
         {
             using MimeMessage? errorMessage = emailRelay.InvalidServerConfiguration(email);
-            await SendErrorMessage(email, errorMessage, cancellationToken);
+            await RejectEmail(email, errorMessage, cancellationToken);
 
-            logger.LogWarning("Could not determine receiver for message #{Id} from {From} to {To}. This message will not be forwarded." +
+            logger.LogWarning("Could not determine receiver for message #{Id} from {From} to {To}. This message will not be forwarded. " +
                 "Please make sure your email provider specifies the receiver in the Received, Envelope-To, or X-Envelope-To header", email.Id, email.From, email.To);
             return;
         }
@@ -56,7 +66,7 @@ protected override async ValueTask ExecuteJob(InboxEmail email, CancellationToke
         if (distributionList == null)
         {
             using MimeMessage? errorMessage = emailRelay.InvalidAlias(email);
-            await SendErrorMessage(email, errorMessage, cancellationToken);
+            await RejectEmail(email, errorMessage, cancellationToken);
 
             logger.LogInformation("No group found with alias {Receiver} for email #{Id} from {From}", email.Receiver, email.Id, email.From);
             return;
@@ -65,7 +75,7 @@ protected override async ValueTask ExecuteJob(InboxEmail email, CancellationToke
         if (email.Header == null)
         {
             using MimeMessage? errorMessage = emailRelay.TooManyHeaders(email);
-            await SendErrorMessage(email, errorMessage, cancellationToken);
+            await RejectEmail(email, errorMessage, cancellationToken);
 
             logger.LogInformation("Email #{Id} from {From} to {Receiver} exceeded the header size limit", email.Id, email.From, email.Receiver);
             return;
@@ -74,7 +84,7 @@ protected override async ValueTask ExecuteJob(InboxEmail email, CancellationToke
         if (email.Body == null)
         {
             using MimeMessage? errorMessage = emailRelay.TooBigMessage(email);
-            await SendErrorMessage(email, errorMessage, cancellationToken);
+            await RejectEmail(email, errorMessage, cancellationToken);
 
             logger.LogInformation("Email #{Id} from {From} to {Receiver} exceeded the body size limit", email.Id, email.From, email.Receiver);
             return;
@@ -96,7 +106,7 @@ protected override async ValueTask ExecuteJob(InboxEmail email, CancellationToke
         return;
     }
 
-    private async ValueTask SendErrorMessage(InboxEmail email, MimeMessage? errorMessage, CancellationToken cancellationToken)
+    private async ValueTask RejectEmail(InboxEmail email, MimeMessage? errorMessage, CancellationToken cancellationToken)
     {
         if (errorMessage != null)
             await emailDelivery.Enqueue(((MailboxAddress)errorMessage.To[0]).Address, errorMessage, email.Id, cancellationToken);

server/Korga/EmailRelay/Entities/InboxEmail.cs

@@ -4,7 +4,7 @@ namespace Korga.EmailRelay.Entities;
 
 public class InboxEmail
 {
-    public InboxEmail(uint? uniqueId, string subject, string from, string? sender, string? replyTo, string to, string? receiver, byte[]? header, byte[]? body)
+    public InboxEmail(uint? uniqueId, string? subject, string? from, string? sender, string? replyTo, string? to, string? receiver, byte[]? header, byte[]? body)
     {
         UniqueId = uniqueId;
         Subject = subject;
@@ -23,11 +23,11 @@ public InboxEmail(uint? uniqueId, string subject, string from, string? sender, s
     public DistributionList? DistributionList { get; set; }
 
     public uint? UniqueId { get; set; }
-    public string Subject { get; set; }
-    public string From { get; set; }
+    public string? Subject { get; set; }
+    public string? From { get; set; }
     public string? Sender { get; set; }
     public string? ReplyTo { get; set; }
-    public string To { get; set; }
+    public string? To { get; set; }
     /// <summary>
     /// The mailbox this email was delivered to via SMTP.
     /// Determined by <c>Received</c>, <c>Envelope-To</c> or <c>X-Envelope-To</c> headers.

server/Korga/EmailRelay/ImapReceiverService.cs

@@ -96,8 +96,10 @@ private async Task QueueEmailForProcessing(ImapClient imap, IMessageSummary mess
                 bodyContent = memoryStream.ToArray();
         }
 
-        string from = message.Headers[HeaderId.From];
-        string to = message.Headers[HeaderId.To];
+        // According to RFC 5322 section 3.6, the Orig-Date and From headers are required
+        // In case of missing headers, the message will be rejected in the next pipeline step
+        string? from = message.Headers[HeaderId.From];
+        string? to = message.Headers[HeaderId.To];
         string? receiver = message.Headers.GetReceiver();
 
         InboxEmail emailEntity = new(