Hi @daisuke_matsuoh and welcome to the Mattermost forums!
This is usually happening when the URL you access mattermost with differs from the configured SiteURL. In your config.json, the SiteURL is set to https://mm.domain.com which is the correct value for the TLS setting, so it should work with this configuration.
Are you trying to invite people using the desktop app or the browser?
Can you see a connection attempt from Mattermost in your postfix logs?