As was stated repeatedly in your previous thread, cat , mail , mailx , and sendmail do not ignore <newline> characters.
If the people to whom you are sending mail are using mail readers that assumes that all text is HTML, that might be what is changing your <newline>s to <space>s. But I have never seen anything that does what you say is happening.
Unless you have replaced the cat , mail or mailx , and sendmail utilities that were loaded when you installed your BSD, Linux, or UNIX operating system and associated utilities; we all know that none of those utilities is ignoring <newline> characters. Any thread that you start that says otherwise is wasting our time.