diff --git a/config.yaml.example b/config.yaml.example
index 4947c72..8887942 100644
--- a/config.yaml.example
+++ b/config.yaml.example
@@ -7,6 +7,7 @@ mail_config:
   smtp_host: localhost
   smtp_port: 25
   base_url: https://motions.cacert.org
+  domain: motions.cacert.org
   notice_mail_address: cacert-board@lists.cacert.org
   vote_notice_mail_address: cacert-board-votes@lists.cacert.org
   notification_sender_address: returns@cacert.org
diff --git a/internal/notifications/mailnotifier.go b/internal/notifications/mailnotifier.go
index 082b578..6ac3b79 100644
--- a/internal/notifications/mailnotifier.go
+++ b/internal/notifications/mailnotifier.go
@@ -1,5 +1,5 @@
 /*
-Copyright 2017-2022 CAcert Inc.
+Copyright 2017-2024 CAcert Inc.
 SPDX-License-Identifier: Apache-2.0
 
 Licensed under the Apache License, Version 2.0 (the "License");
@@ -38,6 +38,7 @@ type MailConfig struct {
 	SMTPHost                  string        `yaml:"smtp_host"`
 	SMTPPort                  int           `yaml:"smtp_port"`
 	SMTPTimeOut               time.Duration `yaml:"smtp_timeout,omitempty"`
+	Domain                    string        `yaml:"message_id_domain"`
 	NotificationSenderAddress string        `yaml:"notification_sender_address"`
 	NoticeMailAddress         string        `yaml:"notice_mail_address"`
 	VoteNoticeMailAddress     string        `yaml:"vote_notice_mail_address"`
@@ -202,10 +203,10 @@ func voteNoticeRecipient(mc *MailConfig) recipientData {
 	}
 }
 
-func motionReplyHeaders(m *models.Motion) map[string][]string {
+func motionReplyHeaders(m *models.Motion, mc *MailConfig) map[string][]string {
 	return map[string][]string{
-		"References":  {fmt.Sprintf("<%s>", m.Tag)},
-		"In-Reply-To": {fmt.Sprintf("<%s>", m.Tag)},
+		"References":  {fmt.Sprintf("<%s@%s>", m.Tag, mc.Domain)},
+		"In-Reply-To": {fmt.Sprintf("<%s@%s>", m.Tag, mc.Domain)},
 	}
 }
 
@@ -247,7 +248,7 @@ func (c *ClosedDecisionNotification) GetNotificationContent(mc *MailConfig) *Not
 			*models.Motion
 		}{Motion: c.Decision},
 		subject:    fmt.Sprintf("Re: %s - %s - finalized", c.Decision.Tag, c.Decision.Title),
-		headers:    motionReplyHeaders(c.Decision),
+		headers:    motionReplyHeaders(c.Decision, mc),
 		recipients: []recipientData{defaultRecipient(mc)},
 	}
 }
@@ -275,14 +276,14 @@ func (n NewDecisionNotification) GetNotificationContent(mc *MailConfig) *Notific
 			UnvotedURL: unvotedURL,
 		},
 		subject:    fmt.Sprintf("%s - %s", n.Decision.Tag, n.Decision.Title),
-		headers:    n.getHeaders(),
+		headers:    n.getHeaders(mc),
 		recipients: []recipientData{defaultRecipient(mc)},
 	}
 }
 
-func (n NewDecisionNotification) getHeaders() map[string][]string {
+func (n NewDecisionNotification) getHeaders(mc *MailConfig) map[string][]string {
 	return map[string][]string{
-		"Message-ID": {fmt.Sprintf("<%s>", n.Decision.Tag)},
+		"Message-ID": {fmt.Sprintf("<%s@%s>", n.Decision.Tag, mc.Domain)},
 	}
 }
 
@@ -309,7 +310,7 @@ func (u UpdateDecisionNotification) GetNotificationContent(mc *MailConfig) *Noti
 			UnvotedURL: unvotedURL,
 		},
 		subject:    fmt.Sprintf("%s - %s", u.Decision.Tag, u.Decision.Title),
-		headers:    motionReplyHeaders(u.Decision),
+		headers:    motionReplyHeaders(u.Decision, mc),
 		recipients: []recipientData{defaultRecipient(mc)},
 	}
 }
@@ -333,7 +334,7 @@ func (d DirectVoteNotification) GetNotificationContent(mc *MailConfig) *Notifica
 			Choice: d.Choice,
 		},
 		subject:    fmt.Sprintf("Re: %s - %s", d.Decision.Tag, d.Decision.Title),
-		headers:    motionReplyHeaders(d.Decision),
+		headers:    motionReplyHeaders(d.Decision, mc),
 		recipients: []recipientData{voteNoticeRecipient(mc)},
 	}
 }
@@ -363,7 +364,7 @@ func (p ProxyVoteNotification) GetNotificationContent(mc *MailConfig) *Notificat
 			Justification: p.Justification,
 		},
 		subject:    fmt.Sprintf("Re: %s - %s", p.Decision.Tag, p.Decision.Title),
-		headers:    motionReplyHeaders(p.Decision),
+		headers:    motionReplyHeaders(p.Decision, mc),
 		recipients: []recipientData{voteNoticeRecipient(mc)},
 	}
 }
@@ -381,7 +382,7 @@ func (w WithDrawMotionNotification) GetNotificationContent(mc *MailConfig) *Noti
 			Name string
 		}{Motion: w.Motion, Name: w.Voter.Name},
 		subject:    fmt.Sprintf("Re: %s - %s", w.Motion.Tag, w.Motion.Title),
-		headers:    motionReplyHeaders(w.Motion),
+		headers:    motionReplyHeaders(w.Motion, mc),
 		recipients: []recipientData{defaultRecipient(mc)},
 	}
 }