diff --git a/.env.test b/.env.test
index fd8b358..fc44bcc 100644
--- a/.env.test
+++ b/.env.test
@@ -7,3 +7,4 @@ POSTGRES_PORT=54320
 POSTGRES_USER=root
 SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN=SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN
 SLACK_SIGNING_SECRET=SLACK_SIGNING_SECRET
+EMAIL_DOMAINS=my-domain.com,ext.my-domain.com
diff --git a/README.md b/README.md
index ee4b1d0..13be23b 100644
--- a/README.md
+++ b/README.md
@@ -202,6 +202,12 @@ Create a `.env` file containing the following variables:
 
     ![OAuth token](docs/assets/slack/oauth-token.png)
 
+- `EMAIL_DOMAINS`
+  Email domains of your organization (separated by comma),
+  for instance `my-domain.com,ext.my-domain.com`.
+  Used to find Slack users from Gitlab username.
+  Note that this means your org has to use the same naming scheme for emails and gitlab username.
+
 If you want Homer to connect to an **external PostgreSQL database**, you can set
 the following variables:
 
diff --git a/src/core/services/slack.ts b/src/core/services/slack.ts
index a7618d7..6787f5a 100644
--- a/src/core/services/slack.ts
+++ b/src/core/services/slack.ts
@@ -8,6 +8,7 @@ import { logger } from './logger';
 const SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN = getEnvVariable(
   'SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN'
 );
+const EMAIL_DOMAINS = getEnvVariable('EMAIL_DOMAINS');
 
 // This client should be used for everything else.
 export const slackBotWebClient = new WebClient(
@@ -94,10 +95,10 @@ export async function fetchSlackUserFromGitlabUser({
 export async function fetchSlackUserFromGitlabUsername(
   username: string
 ): Promise<SlackUser | undefined> {
-  return fetchSlackUserFromEmails([
-    `${username}@manomano.com`,
-    `${username}@prt.manomano.com`,
-  ]);
+  const emails = EMAIL_DOMAINS.split(',').map(
+    (emailDomain) => `${username}@${emailDomain}`
+  );
+  return fetchSlackUserFromEmails(emails);
 }
 
 export async function fetchSlackUserFromId(