If the From: field could only be your domain, it wouldn't make much sense to have a mail form because no one could email you unless they already had access to your domain.
If that's what you want, then there would be no reason to have a public mail form and only people that are supposed to use it should know where it is. If you want a private solution for some reason, you could put it in a password protected directory.
But if you only want email from people you already know, why would they be using a form instead of just emailing you?
You could add code to your form that only accepts email addresses from your domain... but unless it runs through a list of valid email addresses, someone could still enter a fake address like firstname.lastname@example.org if they didn't want you to know who sent it.
Another thing you could do if you only wanted people you know to use the form would be something like a fake anti-spam field. You know the ones that say something like, "What is 5 + 3?" or some other easy question? You could make it so if people don't enter 123456 (or whatever you want) no matter what the question is, it tells them they go it wrong... even though it looks like they didn't.
The To: field is what needs to be secured and what's important to spammers, because you don't want them to put a viagra add in the message field, then add something like this to the To: field: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, etc...
If you have someone that is stalking/harassing you, then the better solution would be to create a new email address and be careful with who you give it to.
If I'm not on the right track, then I must not understand what you're asking. If you could keep people from claiming to be someone else, or forging headers... there's be no more spam.
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