Feature: Add relay config to preserve (keep) original Message-IDs when relaying messages (#515)

Author: axllent

cmd/root.go

@@ -331,6 +331,7 @@ func initConfigFromEnv() {
 	config.SMTPRelayConfig.OverrideFrom = os.Getenv("MP_SMTP_RELAY_OVERRIDE_FROM")
 	config.SMTPRelayConfig.AllowedRecipients = os.Getenv("MP_SMTP_RELAY_ALLOWED_RECIPIENTS")
 	config.SMTPRelayConfig.BlockedRecipients = os.Getenv("MP_SMTP_RELAY_BLOCKED_RECIPIENTS")
+	config.SMTPRelayConfig.PreserveMessageIDs = getEnabledFromEnv("MP_SMTP_RELAY_PRESERVE_MESSAGE_IDS")
 
 	// SMTP forwarding
 	config.SMTPForwardConfigFile = os.Getenv("MP_SMTP_FORWARD_CONFIG")

config/config.go

@@ -237,6 +237,7 @@ type SMTPRelayConfigStruct struct {
 	AllowedRecipientsRegexp *regexp.Regexp // compiled regexp using AllowedRecipients
 	BlockedRecipients       string         `yaml:"blocked-recipients"` // regex, if set prevents relating to these addresses
 	BlockedRecipientsRegexp *regexp.Regexp // compiled regexp using BlockedRecipients
+	PreserveMessageIDs      bool           `yaml:"preserve-message-ids"` // preserve the original Message-ID when relaying
 
 	// DEPRECATED 2024/03/12
 	RecipientAllowlist string `yaml:"recipient-allowlist"`

server/apiv1/application.go

@@ -62,6 +62,9 @@ type webUIConfiguration struct {
 		BlockedRecipients string
 		// Overrides the "From" address for all relayed messages
 		OverrideFrom string
+		// Preserve the original Message-IDs when relaying messages
+		PreserveMessageIDs bool
+
 		// DEPRECATED 2024/03/12
 		// swagger:ignore
 		RecipientAllowlist string
@@ -117,6 +120,8 @@ func WebUIConfig(w http.ResponseWriter, _ *http.Request) {
 		conf.MessageRelay.AllowedRecipients = config.SMTPRelayConfig.AllowedRecipients
 		conf.MessageRelay.BlockedRecipients = config.SMTPRelayConfig.BlockedRecipients
 		conf.MessageRelay.OverrideFrom = config.SMTPRelayConfig.OverrideFrom
+		conf.MessageRelay.PreserveMessageIDs = config.SMTPRelayConfig.PreserveMessageIDs
+
 		// DEPRECATED 2024/03/12
 		conf.MessageRelay.RecipientAllowlist = config.SMTPRelayConfig.AllowedRecipients
 	}

server/apiv1/release.go

@@ -176,13 +176,14 @@ func ReleaseMessage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// generate unique ID
-	uid := shortuuid.New() + "@mailpit"
-	// update Message-ID with unique ID
-	msg, err = tools.SetMessageHeader(msg, "Message-ID", "<"+uid+">")
-	if err != nil {
-		httpError(w, err.Error())
-		return
+	if !config.SMTPRelayConfig.PreserveMessageIDs {
+		// replace the Message-ID header with unique ID
+		uid := shortuuid.New() + "@mailpit"
+		msg, err = tools.SetMessageHeader(msg, "Message-ID", "<"+uid+">")
+		if err != nil {
+			httpError(w, err.Error())
+			return
+		}
 	}
 
 	if err := smtpd.Relay(from, data.To, msg); err != nil {

server/ui-src/components/message/Release.vue

@@ -140,8 +140,8 @@ export default {
 							will be rejected:
 							<code>{{ mailbox.uiConfig.MessageRelay.BlockedRecipients }}</code>
 						</li>
-						<li class="form-text">
-							For testing purposes, a new unique <code>Message-Id</code> will be generated on send.
+						<li v-if="!mailbox.uiConfig.MessageRelay.PreserveMessageIDs" class="form-text">
+							For testing purposes, a new unique <code>Message-ID</code> will be generated on send.
 						</li>
 						<li v-if="mailbox.uiConfig.MessageRelay.OverrideFrom != ''" class="form-text">
 							The <code>From</code> email address has been overridden by the relay configuration to

server/ui/api/v1/swagger.json

@@ -2016,6 +2016,10 @@
               "description": "Overrides the \"From\" address for all relayed messages",
               "type": "string"
             },
+            "PreserveMessageIDs": {
+              "description": "Preserve the original Message-IDs when relaying messages",
+              "type": "boolean"
+            },
             "ReturnPath": {
               "description": "Enforced Return-Path (if set) for relay bounces",
               "type": "string"