php mail function question

Happy new year everyone.

I have a question about the php mail function. My Mac at home is set up as a web server. I use the mail function on a php form handler I am developing on the Mac.

In the recipient field, I entered an email account I have on Hostmonster, the mail went through with a header “Unprivileged User to info”. I am thinking if I can do that, any spammer can do that as well. Is my concern ungrounded? How can I prevent that?

Thanks.

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