Description:
------------
In win32/sendmail.c:SendText the code that examines headers looks for CC and BCC for special treatment. CC is anchored to the beginning of a line but BCC is not.

If there is an X-*bcc header then it will be recognized as a BCC line and
1. PHP will attempt to RCPT TO the header's value
2. The "X-*" will be merged with the next line (if any) when the header is stripped away.

Fix is to tack on a copy of CC's
  && ((pos1 == headers_lc) || (*(pos1-1) == '\n'))
to that one pos1 condition.

Test script:
---------------
$headers = implode("\r\n", [
  "Cc: [email protected]",
  "X-Test-Cc: line 2",
  "X-Test-Aabbcc: line 3",
  "X-Line-4: line 4"
]);
mail("[email protected]", "Subject", "Body", $headers);

Expected result:
----------------
Recipients: [email protected], [email protected]

Cc: [email protected]
X-Test-Cc: line 2
X-Test-Aabbcc: line 3
X-Line-4: line 4

Actual result:
--------------
Recipients: [email protected], [email protected], line 3

Cc: [email protected]
X-Test-Cc: line 2
X-Test-AabX-Line-4: line 4