mysql-php How to filter a user's duplicate submissions when sending a message

Put a text field on the front end that allows the user to enter a mailbox account and then submit the message to the backend for sending tasks

The problem is if the user (malicious) copied into the tens of thousands of mailbox account, and then submitted, and then the user again to the same tens of thousands of copy again, resubmit, so the problem is big

Allow users to submit multiple times, but the data they submit each time is not controllable ....

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Put a text field on the front end that allows the user to enter a mailbox account and then submit the message to the backend for sending tasks

The problem is if the user (malicious) copied into the tens of thousands of mailbox account, and then submitted, and then the user again to the same tens of thousands of copy again, resubmit, so the problem is big

Allow users to submit multiple times, but the data they submit each time is not controllable ....

What is the application scenario, can you describe it?

Is it possible to solve the problem by putting the e-mail address into the database only to prevent duplicate submissions?

Filtering is done at the interface where the message is sent. The user's permission level is divided, the limit of the number of sending objects at different levels is different, and the number of messages sent can be limited.

Record user submissions, limiting the number of times a user submits.

For a user's data (mailbox) to do the deduplication

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