The industry standard for distributing newsletters is either as an email PDF attachment, a URL to that PDF location on a server, or a NewsLetter done in HTML/CSS with a URL link to that website destination in the body of an email. The latter two as an alternative to an email PDF attachment.
I get a newsletter in Apple Mail from Serif Europe (Affinity products) that is based on HTML/CSS styling and shows up nicely formatted — because Apple Mail presents the URL to it on the vendor's server in its local web styling.
You are interpreting the naming of that Pages template (NewsLetter) literally and not as it is intended to be used in a Pages document that is exported to PDF. As I mentioned earlier, no mainstream word processing application is designed to convert word processing template content direct to email formatting (.e.g. .eml), though MS Word, not Pages, can save its content in HTML format.