SMTP Test HTTP 504 (Gateway timeout) for mail address on a different domain

[CodeShakingSheep]

Version:

• listmonk: 4.1.0
• OS: Debian 12

Description of the bug and steps to reproduce:
I have the following setup:

• Listmonk URL https://newsletters.domain1.com
• SMTP 1 connect@domain1.com
• SMTP 2 yes@domain2.com

When I run the SMTP test for SMTP 1 it successfully sends out the test mail. However, when I run the SMTP test for SMTP 2 the POST request https://newsletters.domain1.com/api/settings/smtp/test returns a HTTP 504 response (Gateway timeout) and the test doesn't successfully complete. The weird thing is that both SMTP connection 1 & 2 share the same SMTP host and settings. Just the domain is different. Is it generally not possible to use a SMTP connection with an email address having a different custom domain? Attached you can find some screenshot of the POST request. I'm happy to help with more information if required.

This is the request's payload:

{
"uuid":"87ba2bab-9fb0-4e78-9bc0-d15359f71954",
"enabled":true,
"host":"mail<REDACTED_MAIL_HOST>.com",
"hello_hostname":"",
"port":465,
"auth_protocol":"login",
"username":"yes@domain2.com",
"password":"<REDACTED_PASSWORD>",
"email_headers":[],
"max_conns":10,
"max_msg_retries":2,
"idle_timeout":"15s",
"wait_timeout":"5s",
"tls_type":"STARTTLS",
"tls_skip_verify":false,
"strEmailHeaders":"[]",
"email":"<REDACTED_EMAIL>"
}

Screenshots:

[knadh]

Is it generally not possible to use a SMTP connection with an email address having a different custom domain?

Nothing like that. Any config you enter, listmonk uses the config to create a new SMTP connection. The test endpoint throwing a 504 is really strange. Ideally, the SMTP connection should timeout (if there's a problem) and listmonk should return the appropriate error. Here listmonk itself seems to be timing out.

Do the listmonk logs show anything useful? stdout or Settings -> Logs (lost after a restart)

[CodeShakingSheep]

No, there's nothing in the logs.