Learn how to improve inbox placement in Outlook.
This guide is adapted from Microsoft's article to Improve your spam reputation. For high-volume senders (5,000+ messages per day), see Microsoft's bulk sending requirements for 2025.
Outlook uses email authentication to confirm that you are who you say you are. Microsoft requires bulk senders (5,000+ messages per day) to authenticate with SPF, DKIM, and DMARC, and it's recommended for every sender.
| Authentication | Purpose |
|---|---|
| SPF | Proves you are allowed to send from this domain |
| DKIM | Proves your email originated from you |
| DMARC | Proves you own the domain and instructs how to handle spoofs |
DMARC is an additional authentication method that can build trust and further improve inbox placement.
Action Items
When Outlook or Hotmail filters a message, it records why in the headers. Ask the recipient to view the message headers and look for the fields below. Microsoft documents these in Anti-spam message headers. You can paste the headers into Microsoft's Message Header Analyzer to parse the fields.
Spam Confidence Level (SCL) appears as SCL: in X-Forefront-Antispam-Report. It also often appears in X-MS-Exchange-Organization-SCL. Microsoft stamps a value of -1, or 0 through 9. An SCL of 5 or higher generally indicates the message is considered bad. See Spam confidence level.
| SCL | Meaning | Typical outcome |
|---|---|---|
| -1 | Filtering was bypassed (safe sender, allowlist, or mail flow rule) | Inbox |
| 0 to 1 | Not classified as spam | Inbox |
| 5 to 6 | Classified as spam | Junk |
| 9 | Classified as high confidence spam | Junk or quarantine |
In Microsoft 365 cloud organizations, SCL no longer determines the Spam vs High confidence spam verdict or the action taken. Microsoft says to read the category (CAT) field in X-Forefront-Antispam-Report instead.
| CAT | Meaning |
|---|---|
| SPM | Spam |
| BULK | Bulk |
| HSPM | High confidence spam |
Bulk Complaint Level (BCL) appears in X-Microsoft-Antispam and scores how bulk-like the message is. See Bulk complaint level.
| BCL | Meaning |
|---|---|
| 0 | The message isn't from a bulk sender |
| 1 to 3 | Bulk sender that generates few complaints |
| 4 to 7 | Bulk sender that generates a mixed number of complaints |
| 8 to 9 | Bulk sender that generates a high number of complaints |
Microsoft's default anti-spam policy treats BCL 7 or higher as bulk mail and delivers it to Junk.
SCL 5 and SCL 6 are the same band: Microsoft classified the message as spam. What you do next depends on BCL and CAT, not on which of those two numbers you see.
Read the two scores together. SCL 5 or SCL 6 with BCL 0 means the message was classified as spam but not because it looked like bulk mail, which points at content and domain reputation rather than list quality. A high BCL points the other way, at how the list was built and how recipients are responding. CAT:SPM pairs with the first reading. CAT:BULK pairs with the second.
Action Items
When a message is filtered, the sending IP in the headers is rarely the useful signal. What Microsoft weighs, and what you control, is the sending domain and the history attached to it.
If your messages are being filtered, work on the domain. Checking the shared sending IP against a blocklist is usually a dead end.
Keep one domain or subdomain per sending type so reputation stays separable. See Is it better to send emails from a subdomain or the root domain?.
Action Items
Outlook monitors whether recipients want your mail. Engagement (opens, replies, and moving a message to the Inbox) builds reputation. Complaints, ignored unsubscribes, and bouncing addresses damage it.
Keep spam complaint rates under 0.3%. Don't keep sending if there is no engagement from your recipients, especially if a recipient has requested to unsubscribe or an address is bouncing.
Prevent sending to recipients who:
Ask recipients to add you to their contacts and Safe Senders list. This can be done in Outlook or Outlook.com. See how to add contacts in Outlook.com.
Don't blast to a BCC list. Send separate emails if you are sending to a large number of recipients. Avoid sending too many emails at once. Limits are impacted by historical engagement and sending volume, so reduce the frequency or volume if you start seeing filtering.
For bulk emails, include a clearly visible unsubscribe link and Unsubscribe Headers.
Sending repeatedly to a mailbox you own gives Outlook little to learn from. Real recipients opening, replying, and moving messages out of Junk are the signals that move reputation.
Action Items
Microsoft's filtering leans on history, and a low-volume domain accumulates that history slowly. If you send a few dozen messages a day and that is your steady state rather than a starting point, the warm-up schedules don't apply in the usual way. Those tables start at 150 messages a day and climb. The goal shifts from increasing volume to getting the most signal out of each message.
Action Items
Outlook filters on content as well as reputation. Keep emails as close to plain text as possible, and add a sender name. Set your from like this: "Name <name@example.com>".
Visible links that match the sending domain help. Open or click tracking on sensitive emails (password resets, magic links) often hurts Outlook placement.
Action Items
from addressEmail deliverability is overwhelming. One way to simplify it is to think: what would a phisher do?
Then do the opposite!
Outlook's goal is to only show emails that their users want to see, and malicious emails are at the bottom of the list. Reverse engineer phishing sending habits and consider how you can prove to Outlook at each step that you clearly have no malicious intent.