SMTP configuration works with swaks, but fails with Checkmate (Nodemailer)

[damien-INNOCEAN]

Hi,

I'm running into a strange issue with SMTP notifications in Checkmate.

When I use the exact same SMTP credentials with swaks, emails are sent and received with no problem, both to Gmail and to my company addresses.

When I configure Checkmate (which uses Nodemailer) with these same credentials, I consistently get errors like:

Can't send mail - all recipients were rejected: 550 No Such User Here
Email transporter verification failed

Here are my anonymized SMTP settings

Host: mail.example.com
Port: 587
Username: [email protected]
Password: *****
From: [email protected]

JSON config I used in Checkmate:

{
  "host": "mail.example.com",
  "port": 587,
  "secure": false,
  "auth": {
    "user": "[email protected]",
    "pass": "*****"
  },
  "name": "mail.example.com",
  "pool": false,
  "tls": {
    "rejectUnauthorized": false,
    "ignoreTLS": false,
    "requireTLS": true,
    "servername": "mail.example.com"
  }
}

With swaks, it works every time:

swaks --to [email protected] --from [email protected] --server mail.example.com --port 587 --auth LOGIN --auth-user [email protected] --auth-password '*****' --tls

The email is received by the recipient (Gmail or corporate).

No errors, proper SMTP negotiation, STARTTLS works, etc.

In Checkmate, with the above config:
“Test Email” always fails

Logs show:

Can't send mail - all recipients were rejected: 550 No Such User Here
Email transporter verification failed

Troubleshooting already attempted:

1. Tried "name": "localhost" and "name": "mail.example.com"
2. Tried "requireTLS": true and false
3. Tried "rejectUnauthorized": true and false
4. Tried with/without "servername"
5. Tried port 465 with "secure": true
6. Tried with and without "pool" and the entire "tls" block

Nothing changes the outcome.

Questions

1. Is there a known difference in how Checkmate/NodeMailer constructs the SMTP connection compared to tools like swaks?
2. Are there extra SMTP or TLS options forced by Checkmate that might break with certain servers (Exim/cPanel)?
3. Is there any way to see the full SMTP conversation from the NodeMailer transport as debug output?
4. Any suggestions to fix/workaround this, or a recommended minimal config?

Thanks for your help!

[damien-INNOCEAN]

Update – Problem solved:

I finally found the root cause!
The issue was with the SMTP password: it contained some special characters (*, %, ;, etc.).

When using tools like swaks or other CLI utilities, authentication worked fine.

But when using Checkmate (NodeMailer), the password was not being processed correctly, leading to authentication errors and recipient rejection (550 No Such User Here).

Solution:
I generated a new SMTP password using only letters and numbers (no special characters).
Once I updated the password in Checkmate, everything started working as expected. Test emails send successfully, and notifications are now delivered.

Takeaway:
If you run into similar SMTP issues where your credentials work with some tools but not with Checkmate/NodeMailer, try setting a password with only alphanumeric characters.

Thanks to everyone who took the time to look at this!
Hope this helps someone else down the line.