An email system has 3 main components:

  1. The Mail User Agent (MUA), which is the email client (eg. Mutt, Outlook, Thunderbird)
  2. The Mail Transfer Agent (MTA), which transfers emails from one computer to another (eg. Exim, Sendmail, Postfix)
  3. The Mail Delivery Agent (MDA), which delivers incoming mail to user's inbox (eg. Dovecot, procmail)

Users use a mail client (MUA) to send an email message to their mail server (MTA). The mail server will determine the next hop for the message to take based on any routes that are configured, or typically by looking up the MX record of the recipient domain name. Each time a MTA receives an email message, it adds a Received trace header to the message, allowing for a sequential record of all MTAs that handled the message. If all goes well, the message will end up on a server with a MDA that can deliver the message to the recipient's inbox. The recipient will then use an access protocol such as IMAP, POP3, or Exchange/MAPI to read the email message from their inbox.

Like any other networks, failures may occur in any of the steps in between. MTAs will need to be able to handle problems such as an unavailable MTA. Typically, MTAs are configured to defer any messages on failures and to retry at a later time. A bounce email may be sent to the sender notifying them of the issue.

Troubleshooting[edit]

Emails Going to Junk Folder on Outlook[edit]

For some reason, emails from my server at LiquidWeb are ending up in the Junk folder on Outlook.com hosted accounts (ie. MSN, Hotmail).

Emails are getting set a SCL (Spam Confidence Level) of 5 from the server despite the message having passed DKIM and SPF.

Some other people have the same issue:

More information about Microsoft's filtering:

I'm not sure how to fix. I submitted a request as noted by that thread but don't expect anything to change.