How are you sending the email? Are you sending the text via AJAX to a script in say PHP to send the email?
For example you have: "Hello!" & newline & "Bye!" as your text.
AJAX: post to url("tag", "https://mydomain.com/emailpage.php", URLEncode(replace(text, newline, "<br>")), "POST")
Then when the script receives the post, make sure to URLDecode.