When using PHP’s mail function there is a common question, “Do I need to use CRLF (\r\n) or just a LF (\n) at the end of each line of the additional_headers parameter?” It turns out that if PHP is using postfix’s sendmail program to send email then the answer is you only need LF. This is because postfix’s sendmail program will add the missing CR.
Thus, if you do try to use CRLF while PHP is setup the way described above it will produce an email with headers ending in CRCRLF (\r\r\n) which can cause all sorts of issues. One of these issues that we encountered is that some mail content filtering software will quarantine the message as a potential threat. This appears to be due to the software protecting against the Outlook CR Vulnerability.
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