Cannot send email without a long list of rubbish below the email message

Mac can send mails as Rich Text, with formatting added.

-- Some readers see improved text handling such as alternate font sizes, bold, italic, underline.

-- Other readers see plain characters plus "rubbish"

There are some simple settings that control what you send:

Use plain or rich text in emails in Mail on Mac - Apple Support

depending on HOW LONG the list of rubbish is, that could also be an encoded picture attached to the email.