Free Mail providers such as Hotmail, Yahoo and some other major email
providers, generally send all emails to Junk / Spam email until the
email is confirmed to not be junk, or white-listed. Your customer
normally simply has to white-list, or mark the domain that the email
came from, as safe to receive the newsletter in general emails.
Most people still check their junk/spam mail folder regularly. They may
not treat it with the same level of urgency but will skim 'junk' /
'spam' emails for legitimate ones as they understand from experience
that not all legitimate emails go into their inbox directly.
