Why is Mimecast blocking emails containing Calendly links and other URLs?
Summary
What email marketers say9Marketer opinions
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.
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.
Email marketer from StackExchange shares that Mimecast and other email security services use URL reputation to block emails. If a URL, like Calendly, is frequently associated with spam or malicious activity, it's more likely to be blocked.
Email marketer from EmailGeeks user explains that a domain's reputation is critical for deliverability. If the sending domain or linked domain (like Calendly) has a poor reputation due to spam complaints or blacklisting, email security filters like Mimecast will block the email.
Email marketer from Mailjet Blog shares that using URL shorteners (which Calendly links effectively are) can negatively impact email deliverability because they are frequently abused by spammers, leading to blocks by security filters.
Email marketer from Reddit explains that if a URL in your mail is deemed 'potentially' dangerous or suspicious by URL protection services, your email might be automatically rejected. If your URLs are being blocked, you should reach out to the company.
Email marketer from EmailDrips shares that Calendly has a history of being flagged as spam. Mimecast could block email if any URLs are frequently used in unsolicited email outreach.
Email marketer from Email Vendor Guide shares that Mimecast is known for aggressive filtering and often uses services like Spamhaus. This can lead to legitimate emails with common URLs, like Calendly, being blocked.
Email marketer from Validity shares that Mimecast and other security services use filters based on various factors, including URL reputation, content analysis, and sender reputation, to block potentially harmful emails.
What the experts say7Expert opinions
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.
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.
Expert from Email Geeks recommends testing combinations of the Calendly link and the spamware used to send the email to determine what's triggering Mimecast.
Expert from Word to the Wise mentions calendly but doesn't answer the specific question about why they are blocked by mimecast. The article talks about the importance of getting consent from subscribers so they don't flag your mail as spam which can damage your reputation.
Expert from Email Geeks explains that the HTML portion of the MIME email contains something unusual. When rendered, it leaves a large amount of blank space. She suggests examining the HTML structure and testing the message without the Calendly link.
Expert from Email Geeks explains that the email links to multiple googleusercontent servers and a herokuapp.com domain. It's suggested that malicious content hosted on those sites might trigger the block.
Expert from Spam Resource warns about the dangers of using calendly, the article discusses how easy it is for your email and domain reputation to be tarnished with a single click and the dangers of doing this. If your sending emails are not wanted you may be blacklisted.
What the documentation says4Technical articles
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.
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.
Documentation from Mimecast details that aggressive URL protection is in place to prevent users from clicking on malicious links. If a URL is suspected to be dangerous, Mimecast blocks access, which can affect embedded links like those from Calendly.
Documentation from Cisco details that aggressive URL filtering is used to prevent users from clicking on malicious links. If a URL is categorized as dangerous, users will be blocked, which can affect embedded links.