How to solve hotmail deliverability problems with Salesforce
Outlook.com's spamfilter, which includes hotmail.com and live.com, is sometimes a little bit too smart. Legitimate emails can easily end up in the spam filter. Even the popular site StackOverflow had problems with this.
Figuring out what happens is difficult because the email sometimes doesn't even end up in the junk-folder. It's vanishes in the void.
If you want to be confident that your email will be delivered, you should use a well respected mail provider like Sendgrid or Mailgun to take care of this.
SMTPRelay is middleware between Salesforce your outgoing e-mail server and can be used to greatly improve e-mail deliverability and reliability.