# SMTP: LOGIN authentication failed (STARTTLS/plaintext): sasl: unexpected server challenge

**URL:** <https://forums.nylas.com/t/smtp-login-authentication-failed-starttls-plaintext-sasl-unexpected-server-challenge/980>\
**Category:** Question and Answers\
**Tags:** email-api\
**Created:** [November 10, 2025, 3:25pm UTC](https://forums.nylas.com/t/smtp-login-authentication-failed-starttls-plaintext-sasl-unexpected-server-challenge/980 "2025-11-10T15:25:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![nils](https://avatars.discourse-cdn.com/v4/letter/n/d6d6ee/32.png) [@nils](https://forums.nylas.com/u/nils)\
**Post date:** [November 10, 2025, 3:25pm UTC](https://forums.nylas.com/t/smtp-login-authentication-failed-starttls-plaintext-sasl-unexpected-server-challenge/980/1 "2025-11-10T15:25:31Z")

</div>

Hi there:

We are experiencing SMTP authentication failures when using Custom Authentication:  
Username and password are correct: IMAP Auth works.

Full Error Response:

{

“request\_id”: “1205098503-4a7a663f-368a-4881-824a-7ba423a95a1c”,

“error”: {

```
"type": "grant.invalid_authentication",

"message": "Wrong SMTP username or password provided",

"provider_error": {

  "error": "LOGIN authentication failed (STARTTLS/plaintext): sasl: unexpected server challenge",

  "error_type": "authentication_failed"

}

```

}

}

Status:

- IMAP authentication: ✅ WORKS (same credentials)

- SMTP authentication: ❌ FAILS with SASL error

---

<div class="post-metadata">

**Author:** ![Josias\_M](https://avatars.discourse-cdn.com/v4/letter/j/bc79bd/32.png) [@Josias\_M](https://forums.nylas.com/u/Josias_M)\
**Post date:** [November 10, 2025, 5:22pm UTC](https://forums.nylas.com/t/smtp-login-authentication-failed-starttls-plaintext-sasl-unexpected-server-challenge/980/2 "2025-11-10T17:22:40Z")

</div>

Hello Nils,

We’ve confirmed that this behavior is indeed a bug. The issue has been escalated to our Engineering team for further investigation. I have reached out to you via email, once engineeing has an update expect a follow up via the email.

Best

---

<div class="post-metadata">

**Author:** ![Josias\_M](https://avatars.discourse-cdn.com/v4/letter/j/bc79bd/32.png) [@Josias\_M](https://forums.nylas.com/u/Josias_M)\
**Post date:** [November 10, 2025, 6:18pm UTC](https://forums.nylas.com/t/smtp-login-authentication-failed-starttls-plaintext-sasl-unexpected-server-challenge/980/3 "2025-11-10T18:18:24Z")

</div>