Summary
Google Calendar invites are often marked as junk in Office 365 due to a multifaceted interplay of factors. These include poor sender IP/domain reputation, improper domain authentication (SPF, DKIM, DMARC), problematic content within the .ics file, sudden shifts in sending patterns, high sending volumes, and the configuration of Exchange Online Protection (EOP) anti-spam policies. Also, settings within Outlook desktop client, the use of third-party email routing services, inconsistent filtering rules across domains, and the exploitation of calendar invites by spammers (due to how they bypass traditional filters) play significant roles. Google Workspace provides security settings for calendar invites to prevent phishing, while adhering to the iCalendar data format (RFC 5545) is crucial for correct formatting to ensure proper delivery and receipt. Setting DMARC to p=none isn't harmless either and heavy prospecting from sending domains with poor reputations contributes to this issue.
Key findings
- Sender Reputation: A sender's poor reputation (blocklisting, spam complaints) directly leads to invites being junked.
- Domain Authentication: Improperly configured SPF, DKIM, and DMARC records cause invites to be classified as spam.
- Content Matters: Suspicious URLs, phrasing within .ics files trigger spam filters, leading to junk classification.
- EOP Policies & SCL: EOP anti-spam policies and the Spam Confidence Level (SCL) heavily influence filtering decisions. Misconfigured filters impact invites.
- Sending Patterns: Sudden shifts in sending patterns and high volumes trigger spam filters within O365.
- Client Specific Settings: Individual client spam settings may affect whether an invite is classified as junk.
- Inconsistent Filtering: Inconsistent filtering suggests a problem with how O365 is interpreting invites from different senders.
- Spammers Exploit Invites: Spammers leverage calendar invites to bypass traditional email filters. Volume from unknown sender flags as junk
Key considerations
- Monitor Reputation: Routinely monitor IP/domain reputation and address issues causing poor scores.
- Implement Authentication: Properly configure SPF, DKIM, and DMARC to validate sending source.
- Content Review: Diligently review invite content, eliminating any suspicious links or phrases.
- Control Volume: Avoid sudden high-volume sends, especially to new recipients. Set sending frequency
- EOP Configuration: Carefully configure EOP anti-spam policies to balance aggressiveness and legitimate invite delivery.
- ICS Standard: Comply with the iCalendar data format (RFC 5545) to prevent formatting issues.
- Review Connector Settings: Verify third-party connector settings for enhanced filtering are configured correctly.
- Good invite practices: Good practices such as clear event info and easy opt-out is key.
- Email List Cleaning: Ensuring an accurate and up-to-date email list is used for invites.
What email marketers say
12 marketer opinions
Google Calendar invites can be marked as junk in Office 365 for several reasons, including poor sender reputation (IP address or domain on blocklists), incorrect domain authentication (SPF, DKIM, DMARC), problematic content within the .ics file (suspicious URLs or phrases), sudden changes in sending patterns, high volume of invites sent in a short period, and individual user settings within Outlook. Office 365 uses a combination of content, sender reputation, and user feedback to filter these invites. Spammers often exploit calendar invites due to differing treatment compared to regular emails.
Key opinions
- Sender Reputation: A poor sender reputation, due to being on blocklists or previous spam complaints, can cause calendar invites to be marked as junk.
- Domain Authentication: Incorrect or missing SPF, DKIM, and DMARC records can lead to invites being flagged as spam.
- Content of .ics File: Suspicious URLs or phrases within the .ics file's description can trigger spam filters.
- Sending Volume: Sending a high volume of invites in a short period can trigger spam filters, especially to recipients who don't typically receive them.
- Client-Specific Settings: Individual user settings within Outlook, such as increased spam filtering, can cause invites to be marked as junk for specific users.
- O365 Filtering: Office 365 uses content, sender reputation and user feedback to filter calendar invites.
Key considerations
- Monitor Reputation: Regularly monitor your domain and IP address on blocklists to ensure they are not listed.
- Verify Authentication: Ensure that SPF, DKIM, and DMARC are correctly configured for your sending domain.
- Review Invite Content: Carefully review the content of your calendar invites, avoiding suspicious URLs or phrases.
- Control Sending Volume: Avoid sending a large volume of invites in a short period, especially to new recipients.
- Check User Settings: Advise users to check their individual Outlook settings to ensure spam filtering is not overly aggressive.
- Ensure invite best practices: Ensure you give clear information about the event, avoiding misleading language and ensure recipients can easily unsubscribe or opt-out.
Marketer view
Marketer from Email Geeks explains that DMARC policy set to 'p=none' is not always harmless and can influence filtering. If the issue isn't universal, it's client-specific, suggesting investigation into the common email client, browser, or forwarding method.
26 Nov 2023 - Email Geeks
Marketer view
Email marketer from Mailjet recommends several steps to take to ensure that calendar invites aren't being marked as spam. These are to ensure your sender information is accurate, regularly clean your email list and set a frequency for invites.
18 Aug 2021 - Mailjet
What the experts say
5 expert opinions
Google Calendar invites are marked as junk in Office 365 due to a combination of factors including sender reputation, domain authentication, prospecting practices, and inconsistent filtering across domains. Establishing good practices for calendar invites, such as clear information and easy opt-out, is crucial. Microsoft's Spam Confidence Level (SCL) indicates the likelihood of an invite being spam, and a high SCL contributes to junk mail filtering. Heavy prospecting and poor domain reputation can negatively impact invite deliverability.
Key opinions
- Sender Authentication & Reputation: Calendar invites, like emails, are affected by sender authentication (SPF, DKIM, DMARC) and sending reputation. A poor reputation leads to invites being junked.
- Prospecting Practices: Heavy prospecting by sending domains with poor reputations can cause invites to be flagged.
- Good Invite Practices: Establishing good practices for calendar invites including clear information and an easy opt-out is important to ensure deliverability.
- Domain Consistency: Inconsistent filtering across domains suggests a problem with how O365 is interpreting invites from different senders.
- Spam via Calendar Invites: Calendar invites continue to be a source of spam, with limited tools to combat the issue effectively.
Key considerations
- Implement Authentication: Ensure SPF, DKIM, and DMARC are correctly set up to authenticate the sending domain.
- Monitor Reputation: Regularly check sender reputation and address any issues that may be causing a poor score.
- Review Prospecting: Assess prospecting practices to ensure they are not negatively impacting domain reputation.
- Employ Good Practices: Adopt best practices for calendar invites, focusing on clarity, transparency, and easy opt-out options.
- Review Filtering Rules: Review filtering rules and authentication requirements, ensuring alignment across all sending domains.
Expert view
Expert from Email Geeks suggests that heavy prospecting by the sending domains could be a factor, as those clients might have a poor reputation due to other activities.
12 Sep 2021 - Email Geeks
Expert view
Expert from Word to the Wise explains that it's important to establish good practices for calendar invites. This includes providing clear information about the event, avoiding misleading language, and ensuring recipients can easily unsubscribe or opt-out. Poor practices can lead to invites being flagged as spam.
7 Feb 2022 - Word to the Wise
What the documentation says
5 technical articles
Google Calendar invites can be marked as junk in Office 365 due to anti-spam policies within Exchange Online Protection (EOP) which scan inbound messages for spam and malware. The Spam Confidence Level (SCL) assigned by EOP plays a significant role in determining whether an invite is filtered to junk, with higher SCL values indicating a greater likelihood of spam. Enhanced Filtering for Connectors, when routing email through third-party services, can impact spam filtering accuracy and requires correct configuration. Google Workspace provides security settings for calendar invites to prevent phishing, while adhering to the iCalendar data format (RFC 5545) is crucial for correct formatting to ensure proper delivery and receipt.
Key findings
- EOP Anti-Spam Policies: Exchange Online Protection (EOP) uses anti-spam policies to scan calendar invites for spam and malware. These policies are configurable by administrators to adjust filtering sensitivity.
- Spam Confidence Level (SCL): The Spam Confidence Level (SCL) is a rating assigned by EOP, with higher values indicating a greater likelihood of spam and leading to junk mail filtering.
- Enhanced Filtering: Enhanced Filtering for Connectors in Exchange Online can improve spam filtering accuracy when email is routed through third-party services.
- iCalendar Format: Correct adherence to the iCalendar data format (RFC 5545) is essential for ensuring calendar invites are properly delivered and received.
- Google Workspace Security: Google Workspace provides security settings and best practices to prevent phishing attempts through calendar invites.
Key considerations
- Configure EOP Policies: Administrators should carefully configure EOP anti-spam policies to balance filtering sensitivity with legitimate invite delivery.
- Understand SCL Values: Understand Spam Confidence Level (SCL) values to interpret how Office 365 is classifying invites and adjust policies accordingly.
- Verify Connector Configuration: Ensure Enhanced Filtering for Connectors is correctly configured when using third-party email routing services.
- Adhere to iCalendar Standard: Comply with the iCalendar data format (RFC 5545) to avoid formatting issues that could lead to invites being marked as junk.
- Implement Security Settings: Implement Google Workspace security settings to help prevent phishing attempts through calendar invites.
Technical article
Documentation from Microsoft Learn explains that Exchange Online Protection (EOP) uses anti-spam policies to scan inbound messages (including calendar invites) for spam and malware. Admins can configure these policies to adjust filtering sensitivity.
7 Jun 2023 - Microsoft Learn
Technical article
Documentation from Google Workspace Admin Help specifies security settings and best practices for calendar invites, including ensuring that users are aware of potential phishing attempts through calendar invites. The documentation details that Google's security features help prevent spam.
19 May 2025 - Google Workspace Admin Help
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