diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -3373,11 +3373,15 @@
     'PhabricatorMacroTransactionQuery' => 'applications/macro/query/PhabricatorMacroTransactionQuery.php',
     'PhabricatorMacroTransactionType' => 'applications/macro/xaction/PhabricatorMacroTransactionType.php',
     'PhabricatorMacroViewController' => 'applications/macro/controller/PhabricatorMacroViewController.php',
+    'PhabricatorMailAttachment' => 'applications/metamta/message/PhabricatorMailAttachment.php',
     'PhabricatorMailConfigTestCase' => 'applications/metamta/storage/__tests__/PhabricatorMailConfigTestCase.php',
     'PhabricatorMailEmailHeraldField' => 'applications/metamta/herald/PhabricatorMailEmailHeraldField.php',
     'PhabricatorMailEmailHeraldFieldGroup' => 'applications/metamta/herald/PhabricatorMailEmailHeraldFieldGroup.php',
+    'PhabricatorMailEmailMessage' => 'applications/metamta/message/PhabricatorMailEmailMessage.php',
     'PhabricatorMailEmailSubjectHeraldField' => 'applications/metamta/herald/PhabricatorMailEmailSubjectHeraldField.php',
     'PhabricatorMailEngineExtension' => 'applications/metamta/engine/PhabricatorMailEngineExtension.php',
+    'PhabricatorMailExternalMessage' => 'applications/metamta/message/PhabricatorMailExternalMessage.php',
+    'PhabricatorMailHeader' => 'applications/metamta/message/PhabricatorMailHeader.php',
     'PhabricatorMailImplementationAdapter' => 'applications/metamta/adapter/PhabricatorMailImplementationAdapter.php',
     'PhabricatorMailImplementationAmazonSESAdapter' => 'applications/metamta/adapter/PhabricatorMailImplementationAmazonSESAdapter.php',
     'PhabricatorMailImplementationMailgunAdapter' => 'applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php',
@@ -3443,7 +3447,6 @@
     'PhabricatorMetaMTAApplicationEmailQuery' => 'applications/metamta/query/PhabricatorMetaMTAApplicationEmailQuery.php',
     'PhabricatorMetaMTAApplicationEmailTransaction' => 'applications/metamta/storage/PhabricatorMetaMTAApplicationEmailTransaction.php',
     'PhabricatorMetaMTAApplicationEmailTransactionQuery' => 'applications/metamta/query/PhabricatorMetaMTAApplicationEmailTransactionQuery.php',
-    'PhabricatorMetaMTAAttachment' => 'applications/metamta/storage/PhabricatorMetaMTAAttachment.php',
     'PhabricatorMetaMTAConfigOptions' => 'applications/config/option/PhabricatorMetaMTAConfigOptions.php',
     'PhabricatorMetaMTAController' => 'applications/metamta/controller/PhabricatorMetaMTAController.php',
     'PhabricatorMetaMTADAO' => 'applications/metamta/storage/PhabricatorMetaMTADAO.php',
@@ -9182,11 +9185,15 @@
     'PhabricatorMacroTransactionQuery' => 'PhabricatorApplicationTransactionQuery',
     'PhabricatorMacroTransactionType' => 'PhabricatorModularTransactionType',
     'PhabricatorMacroViewController' => 'PhabricatorMacroController',
+    'PhabricatorMailAttachment' => 'Phobject',
     'PhabricatorMailConfigTestCase' => 'PhabricatorTestCase',
     'PhabricatorMailEmailHeraldField' => 'HeraldField',
     'PhabricatorMailEmailHeraldFieldGroup' => 'HeraldFieldGroup',
+    'PhabricatorMailEmailMessage' => 'PhabricatorMailExternalMessage',
     'PhabricatorMailEmailSubjectHeraldField' => 'PhabricatorMailEmailHeraldField',
     'PhabricatorMailEngineExtension' => 'Phobject',
+    'PhabricatorMailExternalMessage' => 'Phobject',
+    'PhabricatorMailHeader' => 'Phobject',
     'PhabricatorMailImplementationAdapter' => 'Phobject',
     'PhabricatorMailImplementationAmazonSESAdapter' => 'PhabricatorMailImplementationPHPMailerLiteAdapter',
     'PhabricatorMailImplementationMailgunAdapter' => 'PhabricatorMailImplementationAdapter',
@@ -9259,7 +9266,6 @@
     'PhabricatorMetaMTAApplicationEmailQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
     'PhabricatorMetaMTAApplicationEmailTransaction' => 'PhabricatorApplicationTransaction',
     'PhabricatorMetaMTAApplicationEmailTransactionQuery' => 'PhabricatorApplicationTransactionQuery',
-    'PhabricatorMetaMTAAttachment' => 'Phobject',
     'PhabricatorMetaMTAConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorMetaMTAController' => 'PhabricatorController',
     'PhabricatorMetaMTADAO' => 'PhabricatorLiskDAO',
diff --git a/src/applications/audit/editor/PhabricatorAuditEditor.php b/src/applications/audit/editor/PhabricatorAuditEditor.php
--- a/src/applications/audit/editor/PhabricatorAuditEditor.php
+++ b/src/applications/audit/editor/PhabricatorAuditEditor.php
@@ -611,7 +611,7 @@
       $commit->getCommitIdentifier());
 
     $template->addAttachment(
-      new PhabricatorMetaMTAAttachment(
+      new PhabricatorMailAttachment(
         $raw_patch,
         $commit_name.'.patch',
         'text/x-patch; charset='.$encoding));
diff --git a/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php b/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
--- a/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
+++ b/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
@@ -364,7 +364,7 @@
       ->setEvents(array($event))
       ->writeICSDocument();
 
-    $ics_attachment = new PhabricatorMetaMTAAttachment(
+    $ics_attachment = new PhabricatorMailAttachment(
       $ics_data,
       $event->getICSFilename(),
       'text/calendar');
diff --git a/src/applications/differential/editor/DifferentialTransactionEditor.php b/src/applications/differential/editor/DifferentialTransactionEditor.php
--- a/src/applications/differential/editor/DifferentialTransactionEditor.php
+++ b/src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -715,7 +715,7 @@
             $name = pht('D%s.%s.patch', $object->getID(), $diff->getID());
             $mime_type = 'text/x-patch; charset=utf-8';
             $body->addAttachment(
-              new PhabricatorMetaMTAAttachment($patch, $name, $mime_type));
+              new PhabricatorMailAttachment($patch, $name, $mime_type));
           }
         }
       }
diff --git a/src/applications/metamta/management/PhabricatorMailManagementSendTestWorkflow.php b/src/applications/metamta/management/PhabricatorMailManagementSendTestWorkflow.php
--- a/src/applications/metamta/management/PhabricatorMailManagementSendTestWorkflow.php
+++ b/src/applications/metamta/management/PhabricatorMailManagementSendTestWorkflow.php
@@ -193,7 +193,7 @@
       $data = Filesystem::readFile($attachment);
       $name = basename($attachment);
       $mime = Filesystem::getMimeType($attachment);
-      $file = new PhabricatorMetaMTAAttachment($data, $name, $mime);
+      $file = new PhabricatorMailAttachment($data, $name, $mime);
       $mail->addAttachment($file);
     }
 
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAAttachment.php b/src/applications/metamta/message/PhabricatorMailAttachment.php
rename from src/applications/metamta/storage/PhabricatorMetaMTAAttachment.php
rename to src/applications/metamta/message/PhabricatorMailAttachment.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAAttachment.php
+++ b/src/applications/metamta/message/PhabricatorMailAttachment.php
@@ -1,6 +1,6 @@
 <?php
 
-final class PhabricatorMetaMTAAttachment extends Phobject {
+final class PhabricatorMailAttachment extends Phobject {
 
   private $data;
   private $filename;
diff --git a/src/applications/metamta/message/PhabricatorMailEmailMessage.php b/src/applications/metamta/message/PhabricatorMailEmailMessage.php
new file mode 100644
--- /dev/null
+++ b/src/applications/metamta/message/PhabricatorMailEmailMessage.php
@@ -0,0 +1,103 @@
+<?php
+
+final class PhabricatorMailEmailMessage
+  extends PhabricatorMailExternalMessage {
+
+  const MESSAGETYPE = 'email';
+
+  private $fromAddress;
+  private $replyToAddress;
+  private $toAddresses = array();
+  private $ccAddresses = array();
+  private $headers = array();
+  private $attachments = array();
+  private $subject;
+  private $textBody;
+  private $htmlBody;
+
+  public function setFromAddress(PhutilEmailAddress $from_address) {
+    $this->fromAddress = $from_address;
+    return $this;
+  }
+
+  public function getFromAddress() {
+    return $this->fromAddress;
+  }
+
+  public function setReplyToAddress(PhutilEmailAddress $address) {
+    $this->replyToAddress = $address;
+    return $this;
+  }
+
+  public function getReplyToAddress() {
+    return $this->replyToAddress;
+  }
+
+  public function setToAddresses(array $addresses) {
+    assert_instances_of($addresses, 'PhutilEmailAddress');
+    $this->toAddresses = $addresses;
+    return $this;
+  }
+
+  public function getToAddresses() {
+    return $this->toAddresses;
+  }
+
+  public function setCCAddresses(array $addresses) {
+    assert_instances_of($addresses, 'PhutilEmailAddress');
+    $this->ccAddresses = $addresses;
+    return $this;
+  }
+
+  public function getCCAddresses() {
+    return $this->ccAddresses;
+  }
+
+  public function setHeaders(array $headers) {
+    assert_instances_of($headers, 'PhabricatorMailHeader');
+    $this->headers = $headers;
+    return $this;
+  }
+
+  public function getHeaders() {
+    return $this->headers;
+  }
+
+  public function setAttachments(array $attachments) {
+    assert_instances_of($attachments, 'PhabricatorMailAttachment');
+    $this->attachments = $attachments;
+    return $this;
+  }
+
+  public function getAttachments() {
+    return $this->attachments;
+  }
+
+  public function setSubject($subject) {
+    $this->subject = $subject;
+    return $this;
+  }
+
+  public function getSubject() {
+    return $this->subject;
+  }
+
+  public function setTextBody($text_body) {
+    $this->textBody = $text_body;
+    return $this;
+  }
+
+  public function getTextBody() {
+    return $this->textBody;
+  }
+
+  public function setHTMLBody($html_body) {
+    $this->htmlBody = $html_body;
+    return $this;
+  }
+
+  public function getHTMLBody() {
+    return $this->htmlBody;
+  }
+
+}
diff --git a/src/applications/metamta/message/PhabricatorMailExternalMessage.php b/src/applications/metamta/message/PhabricatorMailExternalMessage.php
new file mode 100644
--- /dev/null
+++ b/src/applications/metamta/message/PhabricatorMailExternalMessage.php
@@ -0,0 +1,10 @@
+<?php
+
+abstract class PhabricatorMailExternalMessage
+  extends Phobject {
+
+  final public function getMessageType() {
+    return $this->getPhobjectClassConstant('MESSAGETYPE');
+  }
+
+}
diff --git a/src/applications/metamta/message/PhabricatorMailHeader.php b/src/applications/metamta/message/PhabricatorMailHeader.php
new file mode 100644
--- /dev/null
+++ b/src/applications/metamta/message/PhabricatorMailHeader.php
@@ -0,0 +1,27 @@
+<?php
+
+final class PhabricatorMailHeader
+  extends Phobject {
+
+  private $name;
+  private $value;
+
+  public function setName($name) {
+    $this->name = $name;
+    return $this;
+  }
+
+  public function getName() {
+    return $this->name;
+  }
+
+  public function setValue($value) {
+    $this->value = $value;
+    return $this;
+  }
+
+  public function getValue() {
+    return $this->value;
+  }
+
+}
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
@@ -191,7 +191,7 @@
     return $this;
   }
 
-  public function addAttachment(PhabricatorMetaMTAAttachment $attachment) {
+  public function addAttachment(PhabricatorMailAttachment $attachment) {
     $this->parameters['attachments'][] = $attachment->toDictionary();
     return $this;
   }
@@ -201,7 +201,7 @@
 
     $result = array();
     foreach ($dicts as $dict) {
-      $result[] = PhabricatorMetaMTAAttachment::newFromDictionary($dict);
+      $result[] = PhabricatorMailAttachment::newFromDictionary($dict);
     }
     return $result;
   }
@@ -236,7 +236,7 @@
   }
 
   public function setAttachments(array $attachments) {
-    assert_instances_of($attachments, 'PhabricatorMetaMTAAttachment');
+    assert_instances_of($attachments, 'PhabricatorMailAttachment');
     $this->setParam('attachments', mpull($attachments, 'toDictionary'));
     return $this;
   }
diff --git a/src/applications/metamta/view/PhabricatorMetaMTAMailBody.php b/src/applications/metamta/view/PhabricatorMetaMTAMailBody.php
--- a/src/applications/metamta/view/PhabricatorMetaMTAMailBody.php
+++ b/src/applications/metamta/view/PhabricatorMetaMTAMailBody.php
@@ -155,11 +155,11 @@
   /**
    * Add an attachment.
    *
-   * @param PhabricatorMetaMTAAttachment Attachment.
+   * @param PhabricatorMailAttachment Attachment.
    * @return this
    * @task compose
    */
-  public function addAttachment(PhabricatorMetaMTAAttachment $attachment) {
+  public function addAttachment(PhabricatorMailAttachment $attachment) {
     $this->attachments[] = $attachment;
     return $this;
   }
@@ -187,7 +187,7 @@
   /**
    * Retrieve attachments.
    *
-   * @return list<PhabricatorMetaMTAAttachment> Attachments.
+   * @return list<PhabricatorMailAttachment> Attachments.
    * @task render
    */
   public function getAttachments() {