diff --git a/src/receive_imf.rs b/src/receive_imf.rs
index a0335409c6..e19696a284 100644
--- a/src/receive_imf.rs
+++ b/src/receive_imf.rs
@@ -35,7 +35,7 @@ use crate::peerstate::Peerstate;
 use crate::reaction::{set_msg_reaction, Reaction};
 use crate::securejoin::{self, handle_securejoin_handshake, observe_securejoin_on_other_device};
 use crate::simplify;
-use crate::sql;
+use crate::sql::{self, params_iter};
 use crate::stock_str;
 use crate::sync::Sync::*;
 use crate::tools::{self, buf_compress, remove_subject_prefix};
@@ -1829,9 +1829,6 @@ async fn lookup_chat_or_create_adhoc_group(
     {
         return Ok(Some((new_chat_id, new_chat_id_blocked)));
     }
-    if !allow_creation {
-        return Ok(None);
-    }
     // Partial download may be an encrypted message with protected Subject header. We do not want to
     // create a group with "..." or "Encrypted message" as a subject. The same is for undecipherable
     // messages. Instead, assign the message to 1:1 chat with the sender.
@@ -1854,6 +1851,48 @@ async fn lookup_chat_or_create_adhoc_group(
         .get_subject()
         .map(|s| remove_subject_prefix(&s))
         .unwrap_or_else(|| "👥📧".to_string());
+    let mut contact_ids = Vec::with_capacity(to_ids.len() + 1);
+    contact_ids.extend(to_ids);
+    if !contact_ids.contains(&from_id) {
+        contact_ids.push(from_id);
+    }
+    if let Some((chat_id, blocked)) = context
+        .sql
+        .query_row_optional(
+            &format!(
+                "SELECT c.id, c.blocked
+                FROM chats c INNER JOIN msgs m ON c.id=m.chat_id
+                WHERE m.hidden=0 AND c.grpid='' AND c.name=?
+                AND (SELECT COUNT(*) FROM chats_contacts
+                    WHERE chat_id=c.id)=?
+                AND (SELECT COUNT(*) FROM chats_contacts
+                    WHERE chat_id=c.id
+                    AND contact_id NOT IN ({}))=0
+                ORDER BY m.timestamp DESC",
+                sql::repeat_vars(contact_ids.len()),
+            ),
+            rusqlite::params_from_iter(
+                params_iter(&[&grpname])
+                    .chain(params_iter(&[contact_ids.len()]))
+                    .chain(params_iter(&contact_ids)),
+            ),
+            |row| {
+                let id: ChatId = row.get(0)?;
+                let blocked: Blocked = row.get(1)?;
+                Ok((id, blocked))
+            },
+        )
+        .await?
+    {
+        info!(
+            context,
+            "Assigning message to ad-hoc group {chat_id} with matching name and members."
+        );
+        return Ok(Some((chat_id, blocked)));
+    }
+    if !allow_creation {
+        return Ok(None);
+    }
     create_adhoc_group(
         context,
         mime_parser,
diff --git a/src/receive_imf/tests.rs b/src/receive_imf/tests.rs
index d3842a29a8..f71ad1cf20 100644
--- a/src/receive_imf/tests.rs
+++ b/src/receive_imf/tests.rs
@@ -204,6 +204,71 @@ async fn test_adhoc_group_show_all() {
     assert_eq!(chat::get_chat_contacts(&t, chat_id).await.unwrap().len(), 3);
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_adhoc_groups_merge() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    receive_imf(
+        alice,
+        b"From: bob@example.net\n\
+        To: alice@example.org, claire@example.com\n\
+        Message-ID: <1111@example.net>\n\
+        Date: Sun, 22 Mar 2020 22:37:57 +0000\n\
+        Subject: New thread\n\
+        \n\
+        The first of us should create a thread as discussed\n",
+        false,
+    )
+    .await?;
+    receive_imf(
+        alice,
+        b"From: alice@example.org\n\
+        To: bob@example.net, claire@example.com\n\
+        Message-ID: <2222@example.org>\n\
+        Date: Sun, 22 Mar 2020 22:37:58 +0000\n\
+        Subject: New thread\n\
+        \n\
+        The first of us should create a thread as discussed\n",
+        false,
+    )
+    .await?;
+    let chats = Chatlist::try_load(alice, 0, None, None).await?;
+    assert_eq!(chats.len(), 1);
+    let chat_id = chats.get_chat_id(0)?;
+    assert_eq!(chat_id.get_msg_cnt(alice).await?, 2);
+
+    // If member list doesn't match, threads aren't merged.
+    receive_imf(
+        alice,
+        b"From: bob@example.net\n\
+        To: alice@example.org, claire@example.com, fiona@example.net\n\
+        Message-ID: <3333@example.net>\n\
+        Date: Sun, 22 Mar 2020 22:37:57 +0000\n\
+        Subject: New thread\n\
+        \n\
+        This is another thread, with Fiona\n",
+        false,
+    )
+    .await?;
+    let chats = Chatlist::try_load(alice, 0, None, None).await?;
+    assert_eq!(chats.len(), 2);
+    receive_imf(
+        alice,
+        b"From: bob@example.net\n\
+        To: alice@example.org, fiona@example.net\n\
+        Message-ID: <4444@example.net>\n\
+        Date: Sun, 22 Mar 2020 22:37:57 +0000\n\
+        Subject: New thread\n\
+        \n\
+        This is yet another thread, with Fiona and 0 Claires\n",
+        false,
+    )
+    .await?;
+    let chats = Chatlist::try_load(alice, 0, None, None).await?;
+    assert_eq!(chats.len(), 3);
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_read_receipt_and_unarchive() -> Result<()> {
     // create alice's account
diff --git a/src/sql/migrations.rs b/src/sql/migrations.rs
index 1979b541ef..e9146ae2c4 100644
--- a/src/sql/migrations.rs
+++ b/src/sql/migrations.rs
@@ -1028,6 +1028,15 @@ CREATE INDEX msgs_status_updates_index2 ON msgs_status_updates (uid);
         .await?;
     }
 
+    inc_and_check(&mut migration_version, 121)?;
+    if dbversion < migration_version {
+        sql.execute_migration(
+            "CREATE INDEX chats_index4 ON chats (name)",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?