Emails sent via a Rules action contain arbitrary line breaks
You are most likely up against the hard limit of line length (77) for plain text emails, defined in a Drupal core function _drupal_wrap_mail_line. It quotes RFC 3676 as the reason for this. 

If you want the one line of text you might be able to do it using a HTML formatted email rather than plain text. Alternatively you may want to reformat your email so it suits the available width. The third option of altering core or making a plugin to let you make long lined emails is probably a little overboard.