Summary
Plane ticket emails going to spam in Hotmail, despite passing DKIM in Gmail, is a multifaceted issue. Hotmail employs stricter, proprietary spam filtering, placing greater emphasis on DKIM. This is compounded by potential encoding issues specific to Outlook.com that can cause DKIM verification failures, even when DKIM passes elsewhere. Malformed Reply-To/From headers, sender reputation, and the content and context of the email (ticket confirmations are prime phishing targets) further influence Hotmail's spam filters. A strict DMARC policy in conjunction with DKIM failure can lead to rejection/quarantine. Other factors are the importance of warming up IPs, issues with email rendering, dedicated IPs, and list hygiene. Microsoft possibly changing characters and encoding preferences and varying ISP algorithms adds a high level of complexity.
Key findings
- Hotmail's Stricter System: Hotmail/Outlook.com utilizes a more aggressive spam filtering system compared to Gmail.
- DKIM Failure Sensitivity: Hotmail gives greater weight to DKIM failures, even if SPF passes.
- Encoding Specific to Outlook.com: Character encoding problems are unique to Outlook.com causing DKIM signatures to fail.
- Malformed Headers: Incorrectly formatted Reply-To/From headers increase likelihood of spam filtering.
- Sender Reputation Matters: A poor sender reputation can trigger Hotmail’s filters more readily.
- Content Context Matters: The content and context of emails, e.g., airline tickets, can trigger more aggressive spam filters.
- DMARC Policy: A stricter DMARC policy, combined with DKIM failure, results in emails being rejected or quarantined.
- Varying ISP Policies: Different ISPs and email providers have different levels of strictness regarding authentication and spam filtering. Hotmail's DMARC can be more stringent.
- Random DKIM failures: DKIM failures can occur due to random errors like “0 bit key” or DNS issues
- Microsoft Encoding Preference: Microsoft has a preferenced encoding (ISO vs UTF) that can lead to DKIM failures
Key considerations
- Troubleshoot Encoding: Test email content for encoding problems, such as tabs or characters outside of specified set. Also confirm it is Microsofts preferred encoding type.
- Validate DKIM Setup: Thoroughly verify DKIM configuration, DNS records, and signature verification processes.
- Improve Sender Reputation: Monitor and improve sender reputation by addressing bounce rates, spam complaints, and IP warming.
- Optimize Email Content: Review email content for potential spam triggers, problematic keywords, or phrases.
- Refine Email Rendering: Ensure email rendering is correct, free of suspicious elements, and consistent across clients.
- Correct Header Formatting: Ensure proper formatting of Reply-To and From headers according to RFC specifications.
- Review and Adjust DMARC Policy: Tailor DMARC policy to accommodate Hotmail’s handling of authentication failures.
- IP Warmup: Warm up new IPs and domains to build reputation
What email marketers say
10 marketer opinions
Emails related to plane tickets are landing in Hotmail spam folders despite DKIM passing in Gmail, indicating deliverability issues specific to Hotmail's filtering mechanisms. Potential causes include malformed Reply-To/From headers, DKIM misconfiguration, stricter DMARC policies in Hotmail, poor sender reputation, content triggering spam filters, rendering issues, lack of a dedicated IP, and poor list hygiene.
Key opinions
- Malformed Headers: Incorrectly formatted Reply-To and From headers can trigger Hotmail's spam filters.
- DKIM Configuration: Inconsistent DKIM results across providers suggest a configuration issue. Validate DKIM setup and DNS records.
- Strict DMARC: Hotmail may have stricter DMARC policies; DKIM failures can lead to spam placement.
- Sender Reputation: Poor sender reputation (bounce rates, spam complaints) affects Hotmail more than Gmail.
- Content Triggers: Specific words or phrases associated with spam can trigger Hotmail's filters.
- Rendering Issues: HTML rendering problems may be interpreted as spam by Hotmail.
- Dedicated IP: Lack of a dedicated IP can hinder reputation building, especially for large volume senders.
- List Hygiene: Sending to inactive or unsubscribed users damages sender reputation.
- Warming IP and Domain: Hotmail is strict with new senders - warm IP and domain reputation
Key considerations
- Header Validation: Ensure proper formatting of Reply-To and From headers.
- DKIM Check: Thoroughly verify DKIM configuration and DNS settings.
- DMARC Policy: Understand and adjust DMARC policy, considering Hotmail's handling of failures.
- Reputation Monitoring: Monitor sender score and address any reputation issues promptly.
- Content Review: Review email content for potential spam triggers.
- Rendering Tests: Test emails across different clients (including Hotmail) to ensure proper rendering.
- Dedicated IP Adoption: Consider using a dedicated IP for sending emails.
- List Cleaning: Regularly clean your email list to remove inactive or unsubscribed users.
- IP Warmup: Warm up new IPs and domains to build reputation
Marketer view
Email marketer from StackOverflow mentions that DMARC policies can affect how different email providers handle DKIM failures. If DMARC is set to 'reject' or 'quarantine' and DKIM fails, Hotmail is more likely to send the email to spam, whereas Gmail might be more lenient.
5 Aug 2024 - StackOverflow
Marketer view
Email marketer from Mailchimp explains that maintaining a good sender reputation is crucial for email deliverability. They suggest monitoring your sender score and addressing any issues that could be affecting your reputation, such as high bounce rates or spam complaints. Hotmail might be more sensitive to sender reputation than Gmail.
26 Nov 2023 - Mailchimp
What the experts say
8 expert opinions
Plane ticket emails going to spam in Hotmail despite passing DKIM in Gmail can be attributed to a combination of factors, including Hotmail's stricter spam filtering policies, potential issues with character encoding causing DKIM failures specifically in Outlook.com, format of Reply-To header, sender reputation issues, and the content and context of the email itself. Microsoft potentially altering characters and encoding preferences add further complexity. Different ISPs use different algorithms.
Key opinions
- Encoding Issues: Character encoding problems at Outlook.com can cause DKIM to fail, despite passing elsewhere. Microsoft might swap out a character with something else. There's also preference for Microsoft's preferred encoding.
- DKIM Verification Failures: If the signature didn't verify, then encoding issues is likely the root cause.
- Varying ISP Policies: Different ISPs and email providers have different levels of strictness regarding authentication and spam filtering. Hotmail's DMARC can be more stringent.
- Reply-To and Reputation: Malformed Reply-To headers and poor sender reputation can exacerbate deliverability problems, especially in Hotmail.
- Context and Content: The content and context of emails (e.g., airline tickets) can trigger more aggressive spam filtering.
- Rendering and Formatting: Improperly formatted emails with unusual HTML or suspicious links can be flagged as spam.
- Random DKIM failures: DKIM failures can occur due to random errors like “0 bit key” or DNS issues
Key considerations
- Troubleshoot Encoding: Test email content for encoding problems, such as tabs or characters outside of specified set. Also confirm it is Microsofts preferred encoding type.
- Validate DKIM: Double-check DKIM configuration and DNS records to ensure proper setup and avoid signature verification failures.
- Assess Sender Reputation: Monitor and improve sender reputation by addressing bounce rates and spam complaints.
- Optimize Content: Review email content for potentially problematic keywords or phrases that might trigger spam filters.
- Refine Rendering: Ensure email rendering is correct and free of suspicious elements, such as unusual HTML or excessive images.
- Headers: Format the header correctly, paying close attention to the Reply-To Field.
Expert view
Expert from Email Geeks shares an observation that DKIM fails at Outlook for some brands, even when it's fine everywhere else, but the exact cause remains elusive.
3 Sep 2021 - Email Geeks
Expert view
Expert from SpamResource suggests that issues with the Reply-To address format and sender reputation can play a significant role in deliverability issues. A malformed Reply-To header or a sender with a poor reputation might be more likely to trigger spam filters in Hotmail, especially when combined with a DKIM failure.
14 Sep 2021 - SpamResource
What the documentation says
5 technical articles
Plane ticket emails are directed to spam in Hotmail despite DKIM passing in Gmail due to several factors outlined in the documentation. These include Hotmail's proprietary and more aggressive spam filtering system that strongly considers DKIM failures, especially for transaction emails. A DKIM failure indicates signature verification issues and is treated seriously by Hotmail. Additionally, incorrectly formatted Reply-To headers and strict DMARC policies further contribute to this deliverability issue.
Key findings
- Hotmail's Proprietary System: Hotmail utilizes a unique spam filtering system that is more aggressive than Gmail's.
- DKIM Failure Impact: A DKIM failure, even with SPF pass, negatively impacts deliverability in Hotmail.
- Signature Verification: DKIM failure signifies that the email's signature could not be verified, suggesting tampering.
- Reply-To Header Format: Incorrectly formatted Reply-To headers can trigger spam filters and harm sender reputation.
- Strict DMARC: A strict DMARC policy, combined with DKIM failure, can result in Hotmail rejecting or quarantining messages.
- Email Authentication: Authentication is not a guarantee of deliverability.
Key considerations
- Review Authentication Setup: Ensure DKIM, SPF and DMARC are setup correctly.
- Address DKIM Failures: Investigate and resolve DKIM signature verification issues.
- Correct Header Format: Ensure proper formatting of Reply-To headers according to RFC specifications.
- Monitor DMARC Policy: Review and adjust DMARC policy to align with Hotmail's handling of authentication failures.
Technical article
Documentation from DMARC.org explains how a DMARC policy helps email receivers handle messages that fail authentication checks (SPF and DKIM). A strict DMARC policy, combined with DKIM failure, can lead Hotmail to reject or quarantine messages, while Gmail might have different handling rules based on its own algorithms.
20 Apr 2024 - DMARC.org
Technical article
Documentation from DKIM.org explains that a DKIM failure means the email's signature could not be verified, indicating potential tampering or issues with the signing process. While Gmail might be more lenient, Hotmail may strictly enforce DKIM, especially for transactional emails like plane tickets, which are often targeted by phishing attempts.
1 Apr 2024 - DKIM.org
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