diff --git a/.changeset/fix-default-blocked-email-domains.md b/.changeset/fix-default-blocked-email-domains.md
new file mode 100644
index 0000000000000..5d6bdbe299aff
--- /dev/null
+++ b/.changeset/fix-default-blocked-email-domains.md
@@ -0,0 +1,5 @@
+---
+'@rocket.chat/meteor': patch
+---
+
+Fixes the default blocked email domains list (`Accounts_UseDefaultBlockedDomainsList`) being ignored during registration unless a custom `Accounts_BlockedDomainsList` was also set. The two settings are now independent, so the built-in list of disposable/throwaway domains is enforced on a stock install as intended.
diff --git a/apps/meteor/jest.config.ts b/apps/meteor/jest.config.ts
index 23cf27a6152e4..d0625c39affac 100644
--- a/apps/meteor/jest.config.ts
+++ b/apps/meteor/jest.config.ts
@@ -51,6 +51,7 @@ export default {
 				'<rootDir>/server/api/v1/middlewares/*.spec.ts',
 				'<rootDir>/server/lib/cloud/version-check/**/*.spec.ts',
 				'<rootDir>/server/lib/auth-providers/apple/**.spec.ts',
+				'<rootDir>/server/lib/isEmailDomainBlocked.spec.ts',
 			],
 			coveragePathIgnorePatterns: ['/node_modules/'],
 		},
diff --git a/apps/meteor/server/lib/isEmailDomainBlocked.spec.ts b/apps/meteor/server/lib/isEmailDomainBlocked.spec.ts
new file mode 100644
index 0000000000000..25c6cd0089fca
--- /dev/null
+++ b/apps/meteor/server/lib/isEmailDomainBlocked.spec.ts
@@ -0,0 +1,32 @@
+import { isEmailDomainBlocked } from './isEmailDomainBlocked';
+
+// `0-mail.com` is present in the built-in default list; `gmail.com` is not.
+describe('isEmailDomainBlocked', () => {
+	it('blocks a default-listed domain when the default list is enabled, even with an empty custom list', () => {
+		// Regression: previously the default list was only consulted when the
+		// custom list was non-empty, so on a stock install this returned false.
+		expect(isEmailDomainBlocked('0-mail.com', [], true)).toBe(true);
+	});
+
+	it('does not block a default-listed domain when the default list is disabled', () => {
+		expect(isEmailDomainBlocked('0-mail.com', [], false)).toBe(false);
+	});
+
+	it('blocks a domain on the custom list', () => {
+		expect(isEmailDomainBlocked('evil.example', ['evil.example'], false)).toBe(true);
+	});
+
+	it('does not block a domain that is on neither list', () => {
+		expect(isEmailDomainBlocked('gmail.com', ['evil.example'], true)).toBe(false);
+	});
+
+	it('blocks a default-listed domain regardless of case', () => {
+		// A valid email can carry an upper-case domain (`user@0-MAIL.COM`); the
+		// lower-cased default entry must still match.
+		expect(isEmailDomainBlocked('0-MAIL.COM', [], true)).toBe(true);
+	});
+
+	it('matches the custom list case-insensitively', () => {
+		expect(isEmailDomainBlocked('EVIL.example', ['evil.EXAMPLE'], false)).toBe(true);
+	});
+});
diff --git a/apps/meteor/server/lib/isEmailDomainBlocked.ts b/apps/meteor/server/lib/isEmailDomainBlocked.ts
new file mode 100644
index 0000000000000..bbd2365b58b52
--- /dev/null
+++ b/apps/meteor/server/lib/isEmailDomainBlocked.ts
@@ -0,0 +1,23 @@
+import { emailDomainDefaultBlackList } from './defaultBlockedDomainsList';
+
+/**
+ * Decide whether an email domain should be blocked at registration.
+ *
+ * The admin-configured blocklist (`Accounts_BlockedDomainsList`) and the
+ * built-in default list (`Accounts_UseDefaultBlockedDomainsList`) are
+ * independent controls, so the default list must apply on its own even when no
+ * custom domain has been configured. Previously both checks were gated behind
+ * the custom list being non-empty, so on a stock install the default list was
+ * never consulted.
+ *
+ * Domains are case-insensitive, so the incoming domain is lowercased before
+ * comparison (the default list is stored lowercase); otherwise `user@0-MAIL.COM`
+ * would slip past a listed `0-mail.com`.
+ */
+export const isEmailDomainBlocked = (emailDomain: string, blockList: string[], useDefaultList: boolean): boolean => {
+	const domain = emailDomain.toLowerCase();
+	return (
+		(blockList.length > 0 && blockList.some((blocked) => blocked.toLowerCase() === domain)) ||
+		(useDefaultList && emailDomainDefaultBlackList.includes(domain))
+	);
+};
diff --git a/apps/meteor/server/lib/validateEmailDomain.js b/apps/meteor/server/lib/validateEmailDomain.js
index 8cd8ea9d3c1ef..14cfca4557766 100644
--- a/apps/meteor/server/lib/validateEmailDomain.js
+++ b/apps/meteor/server/lib/validateEmailDomain.js
@@ -4,7 +4,7 @@ import util from 'node:util';
 import { validateEmail } from '@rocket.chat/tools';
 import { Meteor } from 'meteor/meteor';
 
-import { emailDomainDefaultBlackList } from './defaultBlockedDomainsList';
+import { isEmailDomainBlocked } from './isEmailDomainBlocked';
 import { settings } from '../settings';
 
 const dnsResolveMx = util.promisify(dns.resolveMx);
@@ -50,11 +50,7 @@ export const validateEmailDomain = async function (email) {
 			function: 'RocketChat.validateEmailDomain',
 		});
 	}
-	if (
-		emailDomainBlackList.length &&
-		(emailDomainBlackList.indexOf(emailDomain) !== -1 ||
-			(settings.get('Accounts_UseDefaultBlockedDomainsList') && emailDomainDefaultBlackList.indexOf(emailDomain) !== -1))
-	) {
+	if (isEmailDomainBlocked(emailDomain, emailDomainBlackList, settings.get('Accounts_UseDefaultBlockedDomainsList'))) {
 		throw new Meteor.Error('error-email-domain-blacklisted', 'The email domain is blacklisted', {
 			function: 'RocketChat.validateEmailDomain',
 		});