SYMPTOM

Messages sent to remote server from a MailEnable server appear in the recipients junk mail or spam folder. This is happening for messages sent to domains such as Gmail, Outlook.com, Yahoo, etc. 

CAUSES

There are various possibilities as to why messages can be considered as junk by remote servers and delivered to a users junk email folder. Unfortunately it is not always easy from a senders point of view as to why an email goes to the junk folder, and the sender is not aware this has happened. If possible it is worthwhile to firstly check the email headers of the message in the junk folder as there may be some hint there. Check if the "Authentication-Results" and "X-Spam-Status" email headers exist and if there is an indication of the cause in them. SPF and DKIM results are usually listed in the email headers.

Otherwise some of major reasons why emails are junked are:

• No PTR record
• Connecting from a dynamically assigned IP, or a connection classed as a cable/ADSL connection 
• Incorrect sender address
• Email was not sent using TLS
• Invalid/misleading subject line
• An incorrect or missing SPF record for the domain
• No DKIM, or failed DKIM for the sending domain
• Content of the message, such as lots of HTML, images, etc.
• The email has invalid HTML
• The message is being auto-generated by a script or program
• The sending IP address has a poor reputation
• The sending IP address has sent junk email in the past, or users have marked messages from the sending IP as junk before

PREVENTING MESSAGES BEING DELIVERED TO JUNK FOLDER

• Firstly please review the following troubleshooting guides from some major email providers:

  Outlook.com:  http://mail.live.com/mail/troubleshooting.aspx

  Gmail:  https://support.google.com/mail/answer/81126?hl=en

  AOL/Yahoo: https://postmaster.verizonmedia.com

• If emails go to Outlook.com users junk folder, sign up for the Microsoft Junk Email Reporting Program at https://postmaster.live.com/snds/JMRP.aspx to get feedback about your deliveries. You should also check out the other available delivery help from them at https://sendersupport.olc.protection.outlook.com/pm/services.aspx.
• If emails go the Gmail users junk folder, please review https://support.google.com/mail/answer/6227174 and sign up for Postmaster tools to help determine your delivery issues.
• Ensure that the sender address that is configured in the mail client is from a domain that is hosted on the server.
• Enable TLS outbound for the SMTP service.  This is configured under the SMTP Outbound settings.
• Do not configure mailboxes to forward their email to remote servers. If a spam email is forwarded, then your server would be considered the source of spam.
• Ensure that the message being sent does not have a subject or message body that could be mistaken as being spam i.e.: make sure the message has a subject, etc.
• Ensure that a valid PTR record (Reverse DNS record) exists for your IP address.

  For more information on setting up a PTR record please see:

  https://www.mailenable.com/kb/content/article.asp?ID=ME020206

  The following MailEnable tool can perform a check on your domain to see if a valid PTR record exists:

  https://www.mailenable.com/tools/memaillookup.asp

• Publish an SPF record for your domain. Information on how to setup SPF within MailEnable can be found within the following articles:

  https://www.mailenable.com/kb/content/article.asp?ID=ME020345

  MailEnable SPF Documentation

• Implement DKIM, with at least 1024 bit signing. Information on how to setup DKIM for a domain within MailEnable can be found within the MailEnable documentation:

  MailEnable DKIM Configuration

  Learn more about DKIM here:

  https://tools.ietf.org/html/rfc6376