Some clients reported us about the issue when they're trying to login to WebMail using their domain name.
This reason caused when Cloudflare or any kind of CDN is being used on the domain name:webmail port.
Use the hostname:webmail port to login. It will fix the issue immediately. Else, you can use any of your domain that is already pointed to the same server without Cloudflare or any kind of CDN service. Or, you can disable Cloudflare / CDN service used on your domain. It will solve the issue.