Bounce back can only be done by a mail server. It reads the return path in the envelope and send a bounce message back along the return path, to the user who sent it.

The initial mail server (the one you are connecting to) is responsible for parsing the mail that you are sending, and building the envelope correctly. If it doesn't like what it's seeing, it will throw an error and your routine will fail, and then it's up to you to notify the user of the problem. Unless you are writing your own mail server you don't have to worry about bounces. If you are writing a mail server then you shouldn't be sending mail like that.

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Jeremy
I didn't believe in evil until I dated it.