Some possible causes of this problem include:

a) The IP address is not configured with suitable SPF or PTR records.
b) The IP address has been banned and designated as a spammer’s IP address.
c) A large number of emails arrive at the Hotmail/Gmail server from an IP or source that Hotmail/Gmail users mark as Junk or Spam mail.

If any of the IPs were previously blacklisted, it may take some time for Gmail and Hotmail’s spam filtering systems to determine that your email is receiving real messages (approximately 3-7 days). You can also ask your Gmail/Hotmail user (receiver) to mark your emails as Not Junk or Spam at the same time.

For more information, you can check the following links:
Hotmail: [ http://mail.live.com/mail/junkemail.aspx ] –> SmartScreen
Gmail: [ https://support.google.com/mail/answer/81126 ] –> “Delivery”
KB: [ http://gmailblog.blogspot.com/2012/03/learn-why-message-ended-up-in-your-spam.html ]

For Hotmail Users

Emails will still be delivered to the Junk Folder throughout the IP mitigation time or after the IP blacklisting issue: please notice that the Hotmail IP mitigation period will be two days (48 hours).

Emails may end up in Hotmail users’ trash folders during this time. Furthermore, we recommend that Users select the “Not Junk” option in the email folder described above to prevent future messages from being sent to the junk folder and to expedite the mitigation process.

See screenshot below – “Not Junk” option: