SMTP Authentication unsuccessful when using auth_password_file

What did you do? I want to send out Emails and specifying the SMTP Password in a separate file

What did you expect to see? Email should be send to the configured receivers

What did you see instead? Under which circumstances? If an alert fires i got the following error:

alertmanager[2143392]: ts=2024-02-06T08:58:50.982Z caller=notify.go:745 level=warn component=dispatcher receiver=email integration=email[0] aggrGroup={}/{}:{} msg="Notify attempt failed, will retry later" attempts=1 err="*email.loginAuth auth: 535 5.7.3 Authentication unsuccessful"

If i directly configure the auth_password it works.

Environment

System information:

Linux 5.14.0-362.8.1.el9_3.x86_64 x86_64
Alertmanager version:

alertmanager, version 0.26.0 (branch: HEAD, revision: d7b4f0c7322e7151d6e3b1e31cbc15361e295d8d) build user: root@df8d7debeef4 build date: 20230824-11:11:58 go version: go1.20.7 platform: linux/amd64 tags: netgo
Prometheus version:

prometheus, version 2.45.2 (branch: HEAD, revision: 3d6b7e665928a78c4a18675802436fe92f3f4e04) build user: root@3b1b6808f1b0 build date: 20231219-14:07:38 go version: go1.21.5 platform: linux/amd64 tags: netgo,builtinassets,stringlabels

Alertmanager configuration file:

global:
resolve_timeout: 5m
smtp_auth_password_file: '/etc/alertmanager/smtp-password'
route:
receiver: "email"
routes:
- receiver: 'msteams'
  group_wait: 30s
  continue: true
- receiver: 'email'
  group_wait: 30s
group_wait: 30s
group_interval: 5m
repeat_interval: 4h
receivers:
- name: "email"
email_configs:
  - to: 'to address'
    from: 'from address'
    smarthost: 'smtpserver:25'
    auth_username: 'user'
    tls_config:
      insecure_skip_verify: true
- name: "msteams"
msteams_configs:
  - webhook_url: webhook
    send_resolved: true

This is basicly the content of the smtp-password file:

password

Logs:

alertmanager[2143392]: ts=2024-02-06T08:58:50.982Z caller=notify.go:745 level=warn component=dispatcher receiver=email integration=email[0] aggrGroup={}/{}:{} msg="Notify attempt failed, will retry later" attempts=1 err="*email.loginAuth auth: 535 5.7.3 Authentication unsuccessful"

prometheus / alertmanager

SMTP Authentication unsuccessful when using auth_password_file #3710