diff --git a/ProcessMaker/Managers/OauthMailManager.php b/ProcessMaker/Managers/OauthMailManager.php
index 58a7d08b12..fc686f2a28 100644
--- a/ProcessMaker/Managers/OauthMailManager.php
+++ b/ProcessMaker/Managers/OauthMailManager.php
@@ -12,6 +12,7 @@
 use ProcessMaker\Models\EnvironmentVariable;
 use ProcessMaker\Models\Setting;
 use ProcessMaker\Packages\Connectors\Email\EmailConfig;
+use Symfony\Component\Mailer\Transport\Smtp\Auth\XOAuth2Authenticator;
 
 class OauthMailManager extends MailManager
 {
@@ -61,19 +62,13 @@ protected function createSmtpTransport($config)
     {
         $transport = parent::createSmtpTransport($config);
 
-        switch ($this->authMethod) {
-            case 'google':
-                $accessToken = $this->checkForExpiredAccessToken();
-                // Update Authentication Mode
-                $transport->setUsername($this->fromAddress)
-                ->setPassword($accessToken);
-                break;
-            case 'office365':
-                $accessToken = $this->checkForExpiredAccessToken();
-                // Update Authentication Mode
-                $transport->setUsername($this->fromAddress)
+        if (in_array($this->authMethod, ['google', 'office365'], true)) {
+            $accessToken = $this->checkForExpiredAccessToken();
+            // OAuth access tokens must authenticate via XOAUTH2 only.
+            // Trying LOGIN/PLAIN first fails and can poison the SMTP session.
+            $transport->setAuthenticators([new XOAuth2Authenticator()]);
+            $transport->setUsername($this->fromAddress)
                 ->setPassword($accessToken);
-                break;
         }
 
         return $transport;
diff --git a/tests/unit/ProcessMaker/Managers/OauthMailManagerTest.php b/tests/unit/ProcessMaker/Managers/OauthMailManagerTest.php
new file mode 100644
index 0000000000..4dc242360f
--- /dev/null
+++ b/tests/unit/ProcessMaker/Managers/OauthMailManagerTest.php
@@ -0,0 +1,135 @@
+<?php
+
+namespace Tests\Unit\ProcessMaker\Managers;
+
+use ProcessMaker\Managers\OauthMailManager;
+use ReflectionClass;
+use ReflectionMethod;
+use Symfony\Component\Mailer\Transport\Smtp\Auth\LoginAuthenticator;
+use Symfony\Component\Mailer\Transport\Smtp\Auth\XOAuth2Authenticator;
+use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport;
+use Tests\TestCase;
+
+class OauthMailManagerTest extends TestCase
+{
+    private const OAUTH_FROM_ADDRESS = 'oauth-user@example.com';
+
+    public function testGoogleAuthMethodUsesOnlyXoauth2Authenticator()
+    {
+        $transport = $this->createSmtpTransportForAuthMethod('google', 'oauth-access-token');
+
+        $this->assertSame(self::OAUTH_FROM_ADDRESS, $transport->getUsername());
+        $this->assertSame('oauth-access-token', $transport->getPassword());
+        $this->assertAuthenticatorKeywords($transport, ['XOAUTH2']);
+    }
+
+    public function testOffice365AuthMethodUsesOnlyXoauth2Authenticator()
+    {
+        $transport = $this->createSmtpTransportForAuthMethod('office365', 'office-access-token');
+
+        $this->assertSame(self::OAUTH_FROM_ADDRESS, $transport->getUsername());
+        $this->assertSame('office-access-token', $transport->getPassword());
+        $this->assertAuthenticatorKeywords($transport, ['XOAUTH2']);
+    }
+
+    public function testStandardAuthMethodKeepsDefaultAuthenticators()
+    {
+        config([
+            'mail.auth_method' => 'standard',
+            'mail.server_index' => 0,
+            'mail.from.address' => 'standard-user@example.com',
+        ]);
+
+        $manager = new OauthMailManager($this->app);
+        $transport = $this->invokeCreateSmtpTransport($manager, [
+            'host' => 'smtp.example.com',
+            'port' => 587,
+            'username' => 'standard-user@example.com',
+            'password' => 'secret-password',
+            'encryption' => 'tls',
+        ]);
+
+        $keywords = $this->getAuthenticatorKeywords($transport);
+
+        $this->assertContains('LOGIN', $keywords);
+        $this->assertContains('PLAIN', $keywords);
+        $this->assertContains('XOAUTH2', $keywords);
+        $this->assertGreaterThan(1, count($keywords));
+        $this->assertContains(LoginAuthenticator::class, array_map('get_class', $this->getAuthenticators($transport)));
+    }
+
+    private function createSmtpTransportForAuthMethod(string $authMethod, string $accessToken): EsmtpTransport
+    {
+        config([
+            'mail.auth_method' => $authMethod,
+            'mail.server_index' => 0,
+            'mail.from.address' => self::OAUTH_FROM_ADDRESS,
+            'services.gmail' => [
+                'key' => 'gmail-client-id',
+                'secret' => 'gmail-client-secret',
+                'access_token' => $accessToken,
+                'refresh_token' => 'gmail-refresh-token',
+                'expires_in' => time() + 3600,
+            ],
+            'services.office365' => [
+                'tenant_id' => 'tenant-id',
+                'key' => 'office-client-id',
+                'secret' => 'office-client-secret',
+                'access_token' => $accessToken,
+                'refresh_token' => 'office-refresh-token',
+                'expires_in' => now()->addHour()->format('Y-m-d H:i:s'),
+            ],
+        ]);
+
+        $manager = new class($this->app, $accessToken) extends OauthMailManager {
+            private string $testAccessToken;
+
+            public function __construct($app, string $accessToken)
+            {
+                $this->testAccessToken = $accessToken;
+                parent::__construct($app);
+            }
+
+            public function checkForExpiredAccessToken()
+            {
+                return $this->testAccessToken;
+            }
+        };
+
+        return $this->invokeCreateSmtpTransport($manager, [
+            'host' => 'smtp.example.com',
+            'port' => 587,
+            'encryption' => 'tls',
+        ]);
+    }
+
+    private function invokeCreateSmtpTransport(OauthMailManager $manager, array $config): EsmtpTransport
+    {
+        $method = new ReflectionMethod(OauthMailManager::class, 'createSmtpTransport');
+        $method->setAccessible(true);
+
+        return $method->invoke($manager, $config);
+    }
+
+    private function assertAuthenticatorKeywords(EsmtpTransport $transport, array $expectedKeywords): void
+    {
+        $this->assertSame($expectedKeywords, $this->getAuthenticatorKeywords($transport));
+        $this->assertContainsOnlyInstancesOf(XOAuth2Authenticator::class, $this->getAuthenticators($transport));
+    }
+
+    private function getAuthenticatorKeywords(EsmtpTransport $transport): array
+    {
+        return array_map(
+            fn ($authenticator) => $authenticator->getAuthKeyword(),
+            $this->getAuthenticators($transport)
+        );
+    }
+
+    private function getAuthenticators(EsmtpTransport $transport): array
+    {
+        $property = (new ReflectionClass(EsmtpTransport::class))->getProperty('authenticators');
+        $property->setAccessible(true);
+
+        return $property->getValue($transport);
+    }
+}