Problem: SMTP STARTTLS Authentication Fails in Semaphore UI, but Works via curl in Container

Issue

SMTP email alerts fail when configured in Semaphore UI with STARTTLS authentication to Exchange Server. External tools (curl, openssl s_client) confirm TLS and connectivity work correctly, but Semaphore raises authentication errors.

Impact

Configuration

Installation method

Docker

Database

SQLite

Browser

Chrome

Semaphore Version

v2.18.4^0-7ca373d-1779131288

Ansible Version

ansible [core 2.20.5]
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/home/semaphore/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /opt/semaphore/apps/ansible/13.5.0/venv/lib/python3.12/site-packages/ansible
  ansible collection location = /home/semaphore/.ansible/collections:/usr/share/ansible/collections
  executable location = /opt/semaphore/apps/ansible/13.5.0/venv/bin/ansible
  python version = 3.12.14 (main, Aug 13 2026, 19:38:13) [GCC 14.2.0] (/opt/semaphore/apps/ansible/13.5.0/venv/bin/python3)
  jinja version = 3.1.6
  pyyaml version = 6.0.3 (with libyaml v0.2.5)

Logs & errors

Semaphore | time="2026-09-09T14:19:02Z" level=info msg="Task added to queue" task_id=295 task_name="Abruf Switch Infos TEST" Semaphore | time="2026-09-09T14:19:02Z" level=info msg="Set resource locker" context=task_pool task_id=295 task_name="Abruf Switch Infos TEST" Semaphore | time="2026-09-09T14:19:02Z" level=info msg="Task started" context=task_pool task_id=295 task_name="Abruf Switch Infos TEST" Semaphore | time="2026-09-09T14:19:03Z" level=info msg="Task status updated" context=task_logger status=starting task_id=295 Semaphore | time="2026-09-09T14:19:03Z" level=info msg="Task status updated" context=task_logger status=running task_id=295 Semaphore | time="2026-09-09T14:19:07Z" level=warning msg="Failed to run task" context=task_runner error="exit status 4" task_id=295 task_status=running Semaphore | time="2026-09-09T14:19:12Z" level=error msg="530 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM" fields.level=Error Semaphore | time="2026-09-09T14:19:12Z" level=info msg="Task status updated" context=task_logger status=error task_id=295 Semaphore | time="2026-09-09T14:19:12Z" level=info msg="Stopped running task Abruf Switch Infos TEST" task_id=295 ^C

Manual installation - system information

No response

Configuration

{ "sqlite": { "host": "/var/lib/semaphore/database.sqlite" }, "dialect": "sqlite", "tmp_path": "/tmp/semaphore", "cookie_hash": "khash", "cookie_encryption": "cookie_encryption+mbeIvw4c=", "access_key_encryption": "access_key_encryption=", "oidc_providers": { "keycloak": { "display_name": "Magische Anmeldung", "provider_url": "https://sso.stadt.local/realms/stadt", "client_id": "semaphore", "client_secret": "client_secret", "redirect_url": "https://srvsemaphore.stadt.local/api/auth/oidc/keycloak/redirect", "scopes": ["openid", "profile", "email"] } }, "email_alert": true, "email_sender": "[email protected]", "email_host": "mail.stadt.local", "email_port": "587", "email_secure": false, "email_username": "[email protected]", "email_password": "t7+!?5?g!vJ?!!email_password!ug8Q", "email_tls": true, "email_tls_min_version": "1.2" }

Additional information

TLS verification: OK (valid certificate chain)

No client certificate CA names sent Peer signing digest: SHA256 Peer signature type: RSA-PSS Server Temp Key: ECDH, secp384r1, 384 bits

SSL handshake has read 4906 bytes and written 510 bytes Verification: OK

Network connectivity confirmed (no firewall issues)