diff --git a/README.md b/README.md
index 48d98a32..d15901ea 100644
--- a/README.md
+++ b/README.md
@@ -67,7 +67,7 @@ Download the docker-compose.yml, the .env and the setup.sh files:
 
 #### Create your mail accounts
 
-    ./setup.sh email add <email> [<password>]
+    ./setup.sh email add <user@domain> [<password>]
 
 #### Generate DKIM keys
 
diff --git a/target/bin/addmailuser b/target/bin/addmailuser
index 8d881cb1..9e613c81 100755
--- a/target/bin/addmailuser
+++ b/target/bin/addmailuser
@@ -19,6 +19,7 @@ escape() {
 }
 
 [ -z "$USER" ] && { usage; errex "no username specified"; }
+expr index "$USER" "@" >/dev/null || { usage; errex "username must include the domain"; }
 
 grep -qi "^$(escape "$USER")|" $DATABASE 2>/dev/null &&
 	errex "User \"$USER\" already exists"
diff --git a/test/tests.bats b/test/tests.bats
index 3a6a0ad0..47127846 100644
--- a/test/tests.bats
+++ b/test/tests.bats
@@ -1050,6 +1050,11 @@ load 'test_helper/bats-assert/load'
 #
 # accounts
 #
+@test "checking accounts: user_without_domain creation should be rejected since user@domain format is required" {
+  run docker exec mail /bin/sh -c "addmailuser user_without_domain mypassword"
+  assert_failure
+  assert_output --partial "username must include the domain"
+}
 
 @test "checking accounts: user3 should have been added to /tmp/docker-mailserver/postfix-accounts.cf" {
   docker exec mail /bin/sh -c "addmailuser user3@domain.tld mypassword"