SMTP Error: Authentication failure: 5.5.6 Invalid challenge. (Code: 500)

[XurxoMF]

I've Stalwart Mail Server installed and configured.

I'm using my mails with Thunderbird, Blue Mail... and they work fine on port 465.

If I try to use 465 on Roundcube it does nothing when sending a mail. it loads for a while and then fails with no errors, just Could not authenticate.

If I try to use 587 I get the 500 error like in the title.

If I try to use PLAIN or LOGIN(supported and enabled on Stalwart) it fails with Code: 554.

I don't know what to try. I've trued with SMTP username and paswrod %u and %p, 465 and 587, PLAIN and LOGIN...

If someone know what to do, please let me know.

IMAP works fine. I can read mails and everything. The error is when sending a mail only.

There are no error logs on Stalwart, just connection recived from X with user X on port 587/465...

[XurxoMF]

IMAP login log from Stalwart
2025-07-24T19:04:45Z INFO Authentication successful (auth.success) listenerId = "imap", localPort = 143, remoteIp = 185.229.156.90, remotePort = 51308, accountName = "xurxomf", accountId = 97

SMT login log from Stalwart
2025-07-24T19:05:55Z INFO SMTP EHLO command (smtp.ehlo) listenerId = "submission", localPort = 587, remoteIp = 185.229.156.90, remotePort = 55410, domain = "webmail.xurxomf.xyz"

And that's all, error 500...



EDIT: Solved. After 3 weeks of trying when I had free time.
1. Stalwart only allowed PLAIN and LOGIN using TLS on 465. For some reason, 465 and 993 are not working on Roundcube so when using 587, Stalwart was not letting me use PLAIN and LOGIN.... Aded those on Stalwart and problem solved
2. Roundcube was using @mail.xurxomf.xyz instead of @xurxomf.xyz because my username is xurxomf only and it was autocompleting.