Added LocalDomain for setting Mailkit parameter of same name.

Directory.Version.props

@@ -1,5 +1,5 @@
 <Project>
   <PropertyGroup>
-    <VersionPrefix>4.1.0</VersionPrefix>
+    <VersionPrefix>4.1.1</VersionPrefix>
   </PropertyGroup>
 </Project>

src/Serilog.Sinks.Email/LoggerConfigurationEmailExtensions.cs

@@ -54,6 +54,7 @@ public static class LoggerConfigurationEmailExtensions
     /// events passed through the sink. Ignored when <paramref name="levelSwitch"/> is specified.</param>
     /// <param name="levelSwitch">A switch allowing the pass-through minimum level
     /// to be changed at runtime.</param>
+    /// <param name="localDomain">A string specifying the LocalDomain value for MailKit to use, if provided</param>
     /// <returns>
     /// Logger configuration, allowing configuration to continue.
     /// </returns>
@@ -70,7 +71,8 @@ public static LoggerConfiguration Email(
         string? body = null,
         IFormatProvider? formatProvider = null,
         LogEventLevel restrictedToMinimumLevel = LevelAlias.Minimum,
-        LoggingLevelSwitch? levelSwitch = null)
+        LoggingLevelSwitch? levelSwitch = null,
+        string? localDomain = null)
     {
         if (loggerConfiguration == null) throw new ArgumentNullException(nameof(loggerConfiguration));
         if (from == null) throw new ArgumentNullException(nameof(from));
@@ -86,6 +88,7 @@ public static LoggerConfiguration Email(
             ConnectionSecurity = connectionSecurity,
             Credentials = credentials,
             IsBodyHtml = false, // `MessageTemplateTextFormatter` cannot emit valid HTML; the `EmailSinkOptions` overload must be used for this.
+            LocalDomain = localDomain,
         };
 
         if (subject != null)

src/Serilog.Sinks.Email/Sinks/Email/EmailSinkOptions.cs

@@ -97,4 +97,10 @@ public EmailSinkOptions()
     /// Provides a method that validates server certificates.
     /// </summary>
     public System.Net.Security.RemoteCertificateValidationCallback? ServerCertificateValidationCallback { get; set; }
+
+    /// <summary>
+    /// Provides a LocalDomain setting for MailKit in case a custom HELO needs to be used
+    /// </summary>
+    /// <value></value>
+    public string? LocalDomain {get; set;} = null;
 }

src/Serilog.Sinks.Email/Sinks/Email/MailKitEmailTransport.cs

@@ -48,6 +48,11 @@ SmtpClient OpenConnectedSmtpClient()
             smtpClient.ServerCertificateValidationCallback += options.ServerCertificateValidationCallback;
         }
 
+        if (!string.IsNullOrWhiteSpace(options.LocalDomain))
+        {
+            smtpClient.LocalDomain = options.LocalDomain;
+        }
+
         smtpClient.Connect(options.Host, options.Port, options.ConnectionSecurity);
 
         if (options.Credentials != null)

test/Serilog.Sinks.Email.Tests/EmailSinkTests.cs

@@ -152,9 +152,11 @@ public void WorksWithIBatchTextFormatter()
         var sink = new EmailSink(emailConnectionInfo, emailTransport);
 
         using (var emailLogger = new LoggerConfiguration()
-                   .WriteTo.Sink(sink, new BatchingOptions())
+                   .WriteTo.Sink(sink, new BatchingOptions() { EagerlyEmitFirstEvent = false })
                    .CreateLogger())
         {
+            // The EagerlyEmitFirstEvent may occur fast enough that we get multiple sends (linux, especially),
+            // set it false
             emailLogger.Information("Information");
             emailLogger.Warning("Warning");
             emailLogger.Error("<Error>");