diff --git a/core/email_api.php b/core/email_api.php
index 1f2c496..7e02f93 100644
--- a/core/email_api.php
+++ b/core/email_api.php
@@ -801,9 +801,8 @@ function email_send( $p_email_data ) {
 		foreach( $t_email_data->metadata['headers'] as $t_key => $t_value ) {
 			switch( $t_key ) {
 				case 'Message-ID':
-
 					// Overwrite default Message ID
-					$mail->set( 'MessageID', $t_value );
+					$mail->set( 'MessageID', "<$t_value@{$mail->Hostname}>" );
 					break;
 				case 'In-Reply-To':
 					$mail->AddCustomHeader( "$t_key: <{$t_value}@{$mail->Hostname}>" );