SMTP协议具有接收服务器上的邮箱的命令RCPT，如果该邮箱存在，则可能返回ok代码。请注意，某些服务器不会这样做，以防止垃圾邮件发送者扫描有效的地址。

The SMTP protocol has the command RCPT that receives a mailbox on the server and may return an "ok" code if that mailbox exists. Note that some servers don't do this in order to prevent spammers from scanning for valid addresses.

什么电子邮件无限制如下：

What email-unlimited do is the following:

1. 使用电子邮件域查找SMTP服务器


2. 启动与该SMTP服务器的会话，然后发送命令RCPT TO ：


3. 结束会话

如果SMTP服务器在步骤中返回ok代码2，那么他们决定邮箱存在。

If the SMTP server returns an "ok" code in step 2, then they decide that the mailbox exists.