Reply To: Ajax contact form

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above, the OP used IsInjected

That function checks some (not all) forms of newlines that might be used to create new headers, but it does not check for commas at all — it will not catch a spam mailing list.

PHP’s filter_var function will cover all of those cases. And, being a native function, is much faster.

I’m still thinking how it can come to the section of primary of my email.

Have you tried adding a Sender header, as I suggested above?
Your message is most likely being flagged as spam during delivery between MTAs.

OP…. meaning ?

“OP” refers to the “Original Post,” or in this case, “Original Poster,” i.e., you.