Your error happens because Roundcube is trying to use SMTP authentication, but your server on port 25 doesn't support it.

Quick fix: disable authentication in your config:
$rcmail_config['smtp_user'] = '';
$rcmail_config['smtp_pass'] = '';
$rcmail_config['smtp_auth_type'] = null;
This will allow emails to send normally.
It's just a configuration mismatch—like setting up kitchen appliances, everything works best when the settings match the system.
https://applianceworlds.com/product-category/refrigerator/

any solution

Who gave you access to the account, that would be the person to ask.

How can I find the contact information or email address of the email service provider?

It sounds like your an end user of Roundcube you would need to contact the provider of the email service and see what options they have to change the password.

I have an roundcube email address, but its website address is kelgcenter and there is no option to change the password within the email,I checked the web email and there is no plugin for password settings. How can I solve this? This is a web email and I don't have a backend either. I can't find the email administrator either.

Hello I can get help you worked it out alright

The bounce back email should have the reason, a common reason would be if your mail server is blacklisted.

The only one I'm aware of is https://packagist.org/packages/jfcherng-roundcube/cloudview

Just wondering if anyone had any advice.

For the moment my mail is set up to reject nothing, just mark the subject line as spam etc.

I can send and receive emails. Mailbox isn't full.

But there are some that are sent to me that still get returned undelivered to their sender.

Is there anywhere else I need to look to fix this?
Thanks