Why are Google Calendar invites being marked as junk in Office 365?

Key findings

• Specific to invites: The problem often targets only Google Calendar invites with .ics attachments, not regular emails from the same sender.
• SCL score: Affected invites might receive a Spam Confidence Level (SCL) score of 5 in Office 365, indicating Microsoft considers them potentially spam.
• Client-side filtering: If not universal, the issue could be specific to certain recipients' email client setups, such as elevated spam filtering within the Outlook desktop client.
• Phishing vector: Calendar invites, particularly those with embedded links, are a common vector for phishing scams, leading mail providers to apply stricter filtering.

Key considerations

• DMARC policy: Even a DMARC policy set to p=none can influence filtering decisions, and Office 365 expects authentication to align regardless of the DMARC record. For more on this, read our simple guide to DMARC, SPF, and DKIM.
• Sender reputation: The sender's overall domain reputation, potentially impacted by other sending activities like aggressive prospecting, can affect deliverability of all email types, including invites. Learn more in our guide to understanding your email domain reputation.
• Content analysis: Office 365 employs sophisticated filters that scrutinize calendar invite content and attachments (like .ics files) for spam indicators. Issues with emails going to junk in Microsoft Outlook are commonly tied to factors like Spam Confidence Level (SCL) and Bulk Complaint Level (BCL).
• Unsolicited mail: Mail providers are increasingly vigilant against unsolicited calendar invites, due to ongoing phishing attacks that abuse Google Calendar to bypass filters.

Key opinions

• Consistency matters: Marketers frequently question if junking is consistent across domains or if it's client-specific, suggesting a need to analyze patterns of affected recipients.
• Reputation impacts all: There's a strong belief that a sender's overall reputation, possibly influenced by prospecting activities, can play a role in calendar invite filtering, even if regular emails are delivered. This is consistent with advice on why your emails are going to spam in general.
• Attachment scrutiny: The presence of .ics attachments is a key differentiator from regular emails, and marketers consider whether these attachments specifically trigger stricter Office 365 filters.
• Beyond authentication: Even with correct SPF, DKIM, and DMARC alignment, invites can still hit junk, indicating other filtering factors at play. Our article on why authenticated emails go to junk in Microsoft Outlook explores this further.

Key considerations

• Investigate client settings: Check if affected recipients have unique spam filtering rules enabled within their Outlook desktop clients.
• Review prospecting practices: If the sending domain engages in heavy prospecting, review its impact on overall domain reputation. High Bulk Complaint Levels (BCL) can severely impact deliverability.
• Monitor SCL scores: Keep an eye on the Spam Confidence Level assigned by Office 365, as an SCL of 5 or higher indicates a significant risk of junk folder placement. Our guide on fixing Outlook junk mail placement and high SCL scores provides actionable steps.
• Calendar-specific filters: Recognize that mail services apply different filtering logic to calendar invites compared to standard emails, often with a heightened suspicion due to their use in spam. More insights on this behavior are discussed in meeting request spam scenarios.

Key opinions

• DMARC p=none: A DMARC policy set to p=none isn't always harmless and can influence filtering decisions, even if it's not strictly enforced. See our guide on simple DMARC examples.
• Authentication alignment: Office 365 expects email authentication (SPF, DKIM) to align with the sending domain, regardless of whether a DMARC record is published. This is a critical factor for emails landing in Office 365 spam folders.
• Calendar invite spam: A significant amount of spam now arrives via calendar invites, creating a challenge that current filtering tools don't fully address.
• SCL score meaning: An SCL 5 from Microsoft indicates the mail is considered at least 50/50 spam, highlighting the severity of the issue.

Key considerations

• Client-side filtering: Investigate if individual users have increased spam filtering settings within their Outlook desktop client, which could uniquely affect .ics files.
• Reputation assessment: Consider the sender's (and potentially the recipient's) domain reputation. Even if regular emails are clear, historical prospecting or other sending patterns can impact trust for invites. This is a core part of email deliverability issues.
• Calendar spam nature: Understand that spammers use .ics files to embed links without managing complex calendar backends, prompting aggressive filtering by ISPs. Further information on this can be found at BleepingComputer.
• Upstream filters: Be aware that recipients might use third-party filters (like Proofpoint) in conjunction with Office 365, adding another layer of potential junking.

Key findings

• SCL as a core metric: Microsoft's Exchange Online Protection (EOP) and Defender for Office 365 heavily rely on the Spam Confidence Level (SCL) to determine an email's junk status.
• Content and attachment scanning: Calendar invites, especially with .ics attachments, are subjected to stringent content-based filtering to detect malicious or spam-like patterns.
• Authentication standards: Proper implementation and alignment of SPF, DKIM, and DMARC are fundamental for establishing sender trust, though not a guaranteed bypass for all filtering.
• Abuse patterns: Official security advisories frequently highlight the use of calendar invites as a vector for phishing and spam, which informs filter development.

Key considerations

• Monitor SCL: Utilize Office 365 message trace and header analysis to identify the SCL score assigned to specific calendar invites. This provides insight into why they are being marked as junk.
• DMARC policy impact: Understand that even a p=none DMARC policy can still contribute to a lower reputation score if other factors are poor. Our article on how to comply with Outlook's new sender requirements offers more details.
• Content best practices: Ensure calendar invites adhere to best practices for content and formatting to minimize spam triggers. Avoid suspicious links or overly promotional language.
• Authentication validation: Regularly validate SPF, DKIM, and DMARC records to ensure they are correctly configured and aligned, which is fundamental for good deliverability into Microsoft properties.