I thank may be you are referring to this issue which was resolved with the release of 1.6 in 2022.

The password plugin supports (assuming support is complied in your install of PHP) you can set the $config['password_algorithm'] to either "hash-argon2i" or "hash-argon2id".

Does the password plugin support ARGON2I now, after 7 years?
If not, how can i do argon2i on a separate standalone webmail server (without dovecot)?

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.