diff --git a/bun.lockb b/bun.lockb
index a0d3687..1248ee3 100755
Binary files a/bun.lockb and b/bun.lockb differ
diff --git a/package.json b/package.json
index c1b5df2..19ed73a 100644
--- a/package.json
+++ b/package.json
@@ -12,6 +12,7 @@
     "@types/imapflow": "^1.0.19",
     "@types/jsdom": "^21.1.7",
     "@types/mailparser": "^3.4.5",
+    "blakejs": "^1.2.1",
     "imapflow": "^1.0.171",
     "isomorphic-dompurify": "^2.19.0",
     "jsdom": "^25.0.1",
diff --git a/src/config.ts b/src/config.ts
index c5c95de..a340483 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -15,6 +15,7 @@ const maybeConfig: any = {
             type: 'alias',
             template: process.env.CHORUS_THREAD_TEMPLATE,
             idPrefix: 'c.',
+            replyPrefix: '.r.',
         },
     },
     dirs: {
diff --git a/src/mail/read.ts b/src/mail/read.ts
index 3f4faf7..361b81c 100644
--- a/src/mail/read.ts
+++ b/src/mail/read.ts
@@ -10,6 +10,7 @@ import {AsyncCollection} from "../bones/collection/AsyncCollection.ts";
 import {withClient} from "./client.ts";
 import {buildThreadAddressMatcher} from "../threads/id.ts";
 import {htmlReplyPipeline} from "./sanitize.ts";
+import {blake2bHex, blake2sHex} from "blakejs";
 
 export async function getMailboxesToSearch(thread?: string, client?: ImapFlow): Promise<Collection<string>> {
     // There are 2 possibilities for where mail might end up.
@@ -102,12 +103,13 @@ export class MailboxIterable extends Iterable<Message> {
             .map(x => x.address || '')
             .filter(Boolean)
 
-        const thread = recipients.map(addr => this.addressMatcher.exec(addr))
-            .map(result => result?.[1])
+        const addressMatch = recipients.map(addr => this.addressMatcher.exec(addr))
             .filter(Boolean)[0]
 
+        const id = `${this.mailbox}.${message.uid}`
         return {
-            id: `${this.mailbox}.${message.uid}`,
+            id,
+            idHash: blake2sHex(id, undefined, 8),
             date: message.envelope.date,
             from: message.envelope.from[0],
             subject: message.envelope.subject,
@@ -116,7 +118,8 @@ export class MailboxIterable extends Iterable<Message> {
             html: htmlReplyPipeline.apply(source),
             recipients,
             content,
-            thread,
+            thread: addressMatch?.[1],
+            replyToHash: addressMatch?.[2]?.substring(config.mail.threads.replyPrefix.length),
         }
     }
 
diff --git a/src/threads/id.ts b/src/threads/id.ts
index 844fdc1..cae8c02 100644
--- a/src/threads/id.ts
+++ b/src/threads/id.ts
@@ -15,8 +15,21 @@ export function buildThreadAddressMatcher(): RegExp {
     // In the config, the thread ID is represented with the placeholder: %ID%
 
     const idPrefix = escapeRexString(config.mail.threads.idPrefix)
-    const idCapture = `(${idPrefix}[^@]+)`  // first rule of email: don't be as strict as the RFC
+    const idCapture = `(${idPrefix}[^@.]+)`  // first rule of email: don't be as strict as the RFC (special case: IDs after the prefix cannot contain a period)
+
+    const replyPrefix = escapeRexString(config.mail.threads.replyPrefix)
+    const replyCapture = `(${replyPrefix}[^@.]+)?`
+
+    const template = escapeRexString(config.mail.threads.template)
+        .replace('%ID%', idCapture)
+        .replace('%REPLY%', replyCapture)
 
-    const template = escapeRexString(config.mail.threads.template).replace('%ID%', idCapture)
     return new RegExp(`^${template}$`)
 }
+
+export function formatThreadAddress(thread: string, replyToHash?: string): string {
+    let reply = replyToHash ? `${config.mail.threads.replyPrefix}${replyToHash}` : ''
+    return config.mail.threads.template
+        .replace('%ID%', thread)
+        .replace('%REPLY%', reply)
+}
diff --git a/src/threads/refresh.ts b/src/threads/refresh.ts
index 6012ece..f5c23a5 100644
--- a/src/threads/refresh.ts
+++ b/src/threads/refresh.ts
@@ -1,11 +1,12 @@
 import {getMailboxesToSearch, MailboxIterable} from "../mail/read.ts";
 import {withClient} from "../mail/client.ts";
-import type {Message, RootData, ThreadData} from "../types.ts";
+import type {Message, RootData, ThreadComment, ThreadData} from "../types.ts";
 import {AsyncCollection} from "../bones/collection/AsyncCollection.ts";
 import {sha256} from "../bones/crypto.ts";
 import {config} from "../config.ts";
 import { marked } from "marked";
 import {sanitizeHtml} from "../mail/sanitize.ts";
+import {formatThreadAddress} from "./id.ts";
 
 export async function refreshThreadsEntirely(): Promise<void> {
     await withClient(async client => {
@@ -41,6 +42,9 @@ export async function refreshThreadsEntirely(): Promise<void> {
                 comments: [],
             }
 
+            // 3 passes required to make this work (works because objects are by-reference)
+            // Pass 1: create all the ThreadComment instances and make a map by their ID
+            const commentsByHash: {[hash: string]: ThreadComment} = {}
             const messages = messagesByThread[threadId]
             for ( const message of messages ) {
                 if (
@@ -50,7 +54,10 @@ export async function refreshThreadsEntirely(): Promise<void> {
                     threadData.refresh.markers[message.mailbox] = message.modseq
                 }
 
-                threadData.comments.push({
+                commentsByHash[message.idHash] = {
+                    idHash: message.idHash,
+                    replyToHash: message.replyToHash,
+                    replyAddress: formatThreadAddress(threadId, message.idHash),
                     user: {
                         name: message.from.name || '(anonymous)',
                         mailId: sha256(message.from.address!.toLowerCase()),
@@ -60,7 +67,30 @@ export async function refreshThreadsEntirely(): Promise<void> {
                     subject: message.subject,
                     text: message.content,
                     rendered: message.html || sanitizeHtml(await marked(message.content)),
-                })
+                }
+            }
+
+            // Pass 2: Push replies into the replies array of their immediate parent
+            for ( const message of messages ) {
+                if ( !message.replyToHash ) {
+                    continue
+                }
+
+                if ( !commentsByHash[message.replyToHash].replies ) {
+                    commentsByHash[message.replyToHash].replies = []
+                }
+
+                commentsByHash[message.replyToHash].replies!.push(commentsByHash[message.idHash])
+            }
+
+            // Pass 3: push all top-level comments into the ThreadData
+            for ( const idHash of Object.keys(commentsByHash) ) {
+                const comment = commentsByHash[idHash]
+                if ( comment.replyToHash ) {
+                    continue
+                }
+
+                threadData.comments.push(comment)
             }
 
             const json = JSON.stringify(
diff --git a/src/types.ts b/src/types.ts
index 5599797..ef3f782 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -14,6 +14,7 @@ const commentsConfigSchema = z.object({
             type: z.string(),  // fixme : in validation
             template: z.string(),
             idPrefix: z.string(),
+            replyPrefix: z.string(),
         }),
     }),
     dirs: z.object({
@@ -30,6 +31,7 @@ export const castCommentsConfig = (what: unknown): CommentsConfig => {
 
 export type Message = {
     id: string,
+    idHash: string,
     date: Date,
     recipients: string[],
     from: {
@@ -42,6 +44,7 @@ export type Message = {
     mailbox: string,
     modseq: BigInt,
     thread?: string,
+    replyToHash?: string,
 }
 
 export type ThreadUser = {
@@ -51,11 +54,15 @@ export type ThreadUser = {
 }
 
 export type ThreadComment = {
+    idHash: string,
+    replyToHash?: string,
+    replyAddress: string,
     user: ThreadUser,
     date: Date,
     subject: string,
     text: string,
     rendered: string,
+    replies?: ThreadComment[],
 }
 
 export type ThreadData = {