Mailbox unavailable. The server response was: 5.1.1 User unknown, mailboxunavailable

Mailbox unavailable. The server response was: 5.1.1 User unknown, mailboxunavailable

As early as last night, a small feature was implemented in a new project, that is, when a visitor browses a webpage and leaves a message in the message book, the system sends the message content to or CC the specified email address.

Use The code that can normally send emails before, but in The new project, The title error occurs during testing: Mailbox unavailable. The server response was: 5.1.1 User unknown

The email address, account, and password used to send emails are normal accounts. Why is there a User unknown exception?

Originally, this unknown user did not refer to the sender's information, but the recipient was an abnormal user. At that time, the developer initially referred to this mailbox as the default receiving mailbox. This email address has not been created by the network administrator until today.

In fact, Insus. NET has been sending emails for a long time. The following are some notes.
Both the sender, receiver, SMTP server, account, and password are valid.