Summary
Mimecast blocks emails with Calendly links and other URLs due to a combination of factors related to security policies, spam prevention, and URL reputation. These factors include: the association of Calendly links with spam and cold outreach; the use of URL shorteners that are frequently abused by spammers; Mimecast's aggressive filtering practices and use of services like Spamhaus; the presence of unusual HTML structures or links to external servers; and the use of tracking elements. Additionally, a poor domain reputation, high spam scores, potential malware, and being listed on blocklists can trigger Mimecast's filters. Underlying all of these is the need to protect users from malicious links and unwanted emails, often leading to aggressive filtering of anything deemed potentially risky.
Key findings
- Spam Association: Calendly links are frequently used in spam and cold outreach campaigns, leading to their association with unwanted emails.
- URL Reputation: Mimecast uses URL reputation checks to block emails containing links frequently associated with spam or malicious activity.
- Aggressive Filtering: Mimecast is known for its aggressive filtering practices and often uses services like Spamhaus to identify and block potential threats.
- URL Shorteners: URL shorteners, commonly used in Calendly links, are often abused by spammers, leading to deliverability issues.
- Unusual HTML: Unusual HTML structures or suspicious elements within the email's HTML can trigger Mimecast's filters.
- External Links: Linking to multiple external servers or suspicious domains can raise red flags and lead to blocking.
- Tracking Elements: The presence of tracking elements or redirects intended to conceal the email's origin can be identified as deceptive behavior.
- Domain Reputation: A poor domain reputation, stemming from spam complaints or blacklisting, significantly impacts email deliverability and can trigger Mimecast's filters.
- Security Policies: Emails are rejected due to security policies, which may include URL reputation checks, spam scoring, or anti-virus triggers.
- Blocklist Usage: Mimecast uses blocklists like Spamhaus to identify and block spam-related URLs.
- Malware Prevention: URL's found to have malicious code or downloads may be blocked.
Key considerations
- URL Reputation: Regularly check the reputation of URLs included in emails to ensure they are not flagged as spam or malicious.
- HTML Structure: Carefully examine the HTML structure of emails to identify and address any anomalies that may trigger spam filters.
- External Links: Limit the number of external links and ensure that all linked domains are reputable and trustworthy.
- Tracking Elements: Use tracking elements transparently and avoid any deceptive practices that could be flagged as suspicious.
- Domain Reputation: Proactively monitor and maintain a positive domain reputation by adhering to email marketing best practices and avoiding spam complaints.
- Spam Scoring: Optimize email content to achieve low spam scores by avoiding spam trigger words and following best practices for email deliverability.
- Blocklist Monitoring: Monitor blocklists for your sending domain to ensure that your reputation is clean and resolve any listing issues promptly
- Unsolicited Outreach: Always ensure that you have consent before sending any emails.
- Mimecast-Specific Guidance: If problems persist, check Mimecast-specific documentation for troubleshooting
What email marketers say
9 marketer opinions
Mimecast blocks emails containing Calendly links and other URLs primarily due to their association with spam and malicious activity. This is because Calendly links and URL shorteners are frequently used in unsolicited emails and are thus flagged by spam filters. Mimecast's aggressive filtering, combined with URL reputation checks, content analysis, and the use of services like Spamhaus, leads to these blocks. A domain's reputation and the potential danger signaled by URL protection services also contribute to the issue.
Key opinions
- Spam Association: Calendly links are commonly used in spam and cold outreach, leading to negative flagging.
- URL Reputation: Mimecast uses URL reputation to block emails with links frequently associated with spam.
- Aggressive Filtering: Mimecast is known for aggressive filtering practices, often using services like Spamhaus.
- URL Shorteners: URL shorteners (like Calendly links) are often abused by spammers, negatively impacting deliverability.
- Domain Reputation: A poor domain reputation, whether the sending domain or linked domain, triggers Mimecast's filters.
- Security Filters: Mimecast uses security filters to block URLs categorized as malicious or potentially harmful.
- Content Analysis: Security services like Mimecast use content analysis to check if emails appear suspicious and should be blocked.
Key considerations
- Email Reputation: Maintaining a clean email and domain reputation is crucial to avoid being flagged as spam.
- URL Protection: If your URLs are blocked, consider reaching out to Mimecast or other blocking services to resolve the issue.
- Unsolicited Email: Make sure you only send wanted and solicited mail to build a positive domain and email reputation.
- Filter Services: URL are used to categorize potential threats so make sure to keep your email clean.
- Analyze all urls: Any URL can be a risk factor when sending email so check the reputatation of those before sending.
Marketer view
Email marketer from SendGrid explains that security filters block URLs that are categorized in malicious content. Email security filters like Mimecast commonly block these URLs to protect the recipient.
21 Jan 2024 - SendGrid
Marketer view
Email marketer from Reddit explains that Calendly links are often used in spam and cold outreach, causing them to be flagged by spam filters like Mimecast due to their association with unsolicited emails.
15 Dec 2021 - Reddit
What the experts say
7 expert opinions
Mimecast's blocking of emails containing Calendly links and other URLs can stem from several factors. Unusual HTML structures, links to multiple external servers (including googleusercontent and herokuapp.com), and the presence of tracking elements like the 'apollo_tracker' may trigger blocks. The use of redirects to hide the origin of spamware is another potential cause. Domain and email reputation is key, and unsolicited emails may lead to blacklisting. Testing combinations of Calendly links and spamware usage is advised to pinpoint the trigger. While obtaining consent from subscribers and not sending unwanted emails are essential for positive reputation.
Key opinions
- Unusual HTML: Unusual structures or excessive blank space in the HTML portion of emails may trigger Mimecast.
- External Links: Links to multiple external servers (e.g., googleusercontent, herokuapp.com) can raise suspicion.
- Tracking Pixels: The presence of tracking elements and redirects can be flagged as deceptive behavior.
- Spamware: The use of spamware and methods to conceal its origin contributes to blocking.
- Apollo Tracker: The presence of the 'apollo_tracker' or domains associated with Apollo might lead to blocks, particularly in cold outreach.
- Domain Reputation: If your emails are unwanted, your emails may be blocked and your domain reputation is impacted
Key considerations
- HTML Structure: Examine the HTML structure of your emails for anomalies.
- Link Testing: Test messages without Calendly links and other suspicious URLs to isolate the cause.
- Content Hosting: Evaluate the content hosted on linked domains (e.g., herokuapp.com) for malicious content.
- Combination Testing: Test combinations of Calendly links and spamware usage to identify triggers.
- Consent: Make sure to gain subscribers consent and send only wanted email
Expert view
Expert from Email Geeks explains that the heroku URL redirects to an aptracking1.com hosted URL, which then redirects to a transparent pixel on Amazon S3. This is a method to hide the origin of the email and the B2B spamware it is sent from.
10 Dec 2023 - Email Geeks
Expert view
Expert from Email Geeks suggests that Mimecast may be blocking emails due to the presence of the "apollo_tracker" or domains associated with Apollo, especially if it's unsolicited cold outreach.
23 Nov 2024 - Email Geeks
What the documentation says
4 technical articles
Mimecast blocks emails containing Calendly links and other URLs due to security policies and aggressive URL protection. These policies include URL reputation checks, spam scoring, and anti-virus triggers. Mimecast uses blocklists like Spamhaus to filter out spam, blocking URLs found in spam campaigns or those considered dangerous to prevent users from clicking malicious links. Cisco systems has similair processes in place.
Key findings
- Security Policies: Emails are rejected due to security policies that include URL reputation checks, spam scoring, and anti-virus triggers.
- Spamhaus Blocklists: Mimecast uses Spamhaus blocklists to filter out spam, which can lead to URLs in spam campaigns being blocked.
- Aggressive URL Protection: Mimecast has aggressive URL protection to prevent users from clicking on malicious links, potentially blocking Calendly links.
- Aggressive URL Filtering: Cisco also utilizes aggressive URL filtering
Key considerations
- URL Reputation: Check the reputation of URLs before including them in emails to avoid being flagged as spam.
- Spam Scoring: Ensure emails have low spam scores by following best practices for content and sending behavior.
- Anti-Virus: Scan email content and attachments for viruses or malware to prevent triggering anti-virus filters.
- Blocklist Checks: Monitor whether your URLs or sending domains are listed on blocklists like Spamhaus and take steps to be removed if listed.
Technical article
Documentation from Mimecast explains that emails are rejected due to security policies, which may include URL reputation checks, spam scoring, or anti-virus triggers. If a link's signature is considered a virus or the spam score exceeds the maximum threshold, it will be blocked.
19 Nov 2021 - Mimecast
Technical article
Documentation from Spamhaus explains that their blocklists are used by many email security services, including Mimecast, to filter out spam. URLs found in spam campaigns are often listed, leading to blocks. Mimecast uses Spamhaus.
27 Mar 2022 - Spamhaus
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