How does WordPress restrict user registration of available mailbox domain name suffixes?

Then you can try the User Domain Whitelist or cm e-Mail Blacklist plug-in.

Both functions are similar. User Domain Whitelist allows you to use the "Whitelist" or "blacklist" mode. For example, when you select the "Whitelist" mode, only the email domain names added to the whitelist can be registered. When the "blacklist" mode is selected, domain names in the blacklist list are not allowed.

In the same way, you can set the whitelist and Blacklist for cm e-Mail Blacklist. You can also test whether the configuration takes effect.

Search User Domain Whitelist or cm e-Mail Blacklist on the background plug-in installation interface to install the agent online.