diff --git a/src/mimeparser.rs b/src/mimeparser.rs
index 6f7a66544..ec7b098a3 100644
--- a/src/mimeparser.rs
+++ b/src/mimeparser.rs
@@ -825,7 +825,11 @@ fn update_gossip_peerstates(
                 })));
             }
 
-            if recipients.as_ref().unwrap().contains(&header.addr) {
+            if recipients
+                .as_ref()
+                .unwrap()
+                .contains(&header.addr.to_lowercase())
+            {
                 let mut peerstate = Peerstate::from_addr(context, &context.sql, &header.addr);
                 if let Some(ref mut peerstate) = peerstate {
                     peerstate.apply_gossip(header, message_time);
@@ -981,7 +985,7 @@ fn get_attachment_filename(mail: &mailparse::ParsedMail) -> Result<String> {
     Ok(desired_filename)
 }
 
-// returned addresses are normalized.
+// returned addresses are normalized and lowercased.
 fn get_recipients<S: AsRef<str>, T: Iterator<Item = (S, S)>>(headers: T) -> HashSet<String> {
     let mut recipients: HashSet<String> = Default::default();
 
@@ -993,11 +997,11 @@ fn get_recipients<S: AsRef<str>, T: Iterator<Item = (S, S)>>(headers: T) -> Hash
                 for addr in addrs.iter() {
                     match addr {
                         mailparse::MailAddr::Single(ref info) => {
-                            recipients.insert(addr_normalize(&info.addr).into());
+                            recipients.insert(addr_normalize(&info.addr).to_lowercase());
                         }
                         mailparse::MailAddr::Group(ref infos) => {
                             for info in &infos.addrs {
-                                recipients.insert(addr_normalize(&info.addr).into());
+                                recipients.insert(addr_normalize(&info.addr).to_lowercase());
                             }
                         }
                     }
diff --git a/test-data/message/mail_with_cc.txt b/test-data/message/mail_with_cc.txt
index 02b749ffd..3f9254e47 100644
--- a/test-data/message/mail_with_cc.txt
+++ b/test-data/message/mail_with_cc.txt
@@ -9,6 +9,6 @@ message-id: <2dfdbde7@example.org>
 Date: Sat, 14 Sep 2019 19:00:13 +0200
 From: lmn <x@tux.org>
 To: abc <abc@bcd.com>
-CC: def <def@def.de>
+CC: def <Def@def.de>
 
 hi