Why would Mimecast block a legitimate service like Calendly?

While Calendly is a legitimate service, its URLs are frequently used in unsolicited cold outreach. This widespread misuse by spammers can degrade the domain's reputation, leading security gateways like Mimecast to implement broad blocking policies (or blacklists) to protect their users from perceived spam. The blocking might not be due to an inherent flaw in Calendly itself, but rather its association with low-quality sending practices.

What steps can I take to prevent Mimecast from blocking my emails with Calendly links?

To reduce the risk of blocking, you can: 1) use a custom redirect domain for your Calendly links, 2) carefully review your email's HTML for suspicious elements or tracking pixels, 3) send test emails with variations (e.g., without the link, without other tracking) to pinpoint the exact trigger, and 4) leverage Mimecast's Sender Feedback form for review.

Can Mimecast block my own outbound emails, or does it only affect inbound mail?

Yes, Mimecast employs both inbound and outbound filtering. This means that if your organization uses Mimecast , your own outbound emails could be blocked by your company's security policies before they ever reach the recipient's server. This highlights the importance of internal compliance with email security best practices.

Why is Mimecast blocking emails containing Calendly links and other URLs? - Troubleshooting - Email deliverability - Knowledge base

Q: What does a "554 Email rejected due to security policies" bounce message mean?

A 554 Email rejected due to security policies bounce message from Mimecast is a generic error that indicates something in your email triggered one of their anti-spam or anti-virus signatures. This could be due to a specific URL, but it can also be triggered by suspicious HTML formatting, hidden tracking pixels, or content that resembles known malicious patterns. It's often a broader content-based block, not just a link-specific one.

Emails containing

Calendly links, or other scheduling and content sharing URLs, are sometimes blocked by email security solutions like

Mimecast. This can be frustrating, especially when these links are legitimate parts of your business communications. The underlying reasons are complex, involving Mimecast’s aggressive filtering, the reputation of the linked domains, and even the structure of your email content itself.

When an email is rejected with a 554 Email rejected due to security policies bounce message, it often indicates that Mimecast's (or another security solution's) filters have identified something suspicious. This isn't always about the specific URL, but can involve broader email attributes or hidden tracking elements. We'll explore why this happens and what steps you can take to prevent these blocks.

How Mimecast’s URL protection works

Mimecast is a leading email security provider that employs sophisticated mechanisms to protect its users from spam, malware, and phishing attacks. One of its key features is URL Protect, which scans and modifies links within emails to prevent users from accessing malicious websites. This protection can sometimes lead to legitimate URLs being flagged if they are associated with a history of abuse or exhibit suspicious patterns.

The platform analyzes various factors, including the domain's reputation, historical data, and the context in which the URL appears. If Mimecast's systems detect a URL that has been used in spam or phishing campaigns, even if it is from a legitimate service, it may trigger a block. This aggressive filtering is designed to prioritize security, but it can inadvertently impact legitimate senders.

Mimecast's Targeted Threat Protection, particularly its URL Protect component, can rewrite URLs in emails to ensure their safety. This process involves scanning links for malicious content before delivery. You can read more about it on the Mimecast Support site. If a URL is deemed risky, Mimecast may block the email, strip the link, or warn the recipient.

Why Calendly links can be problematic

While Calendly is a widely used and legitimate scheduling tool, its popularity has also made it a target for spammers. Some bad actors exploit these services for cold outreach, sending unsolicited emails with Calendly links in attempts to book meetings. This widespread misuse can degrade the overall reputation of Calendly URLs, leading email security providers like Mimecast to view them with increased suspicion.

Mimecast’s filters are designed to identify patterns associated with spam and phishing. If a high volume of spam emails contain Calendly links, Mimecast may implement a blanket block or blocklist (or blacklist) on those URLs to protect its users. This means even legitimate emails containing Calendly links might be caught in the crossfire, especially if they resemble known spam campaigns. Businesses that primarily rely on B2B communications are particularly susceptible to this due to the prevalence of Mimecast in enterprise environments.

Another factor is whether the Calendly link is wrapped by an Email Service Provider's (ESP) click-tracking domain. If you send Calendly links directly from services like

Google Workspace or

Microsoft 365 (without an ESP), the bare

Calendly URL might be more susceptible to filtering. ESPs often use their own tracking domains, which can help mask the direct

Calendly link from initial scanning, though this is not a guaranteed solution. Learn more about why email filters modify links.

Beyond Calendly: other problematic URLs and content

The problem extends beyond Calendly. Mimecast and similar security solutions (like

Proofpoint) can block any URL if it's perceived as a threat. This often comes down to URL reputation. If a domain has been recently registered, has a suspicious domain name, or has been previously associated with spam or malicious activity (even inadvertently), it can land on internal blocklists (or blacklists) used by email security gateways.

The 554 Email rejected due to security policies error message itself can be generic. It might signify that something in your message triggered a broader anti-virus or anti-spam signature within Mimecast. This isn't necessarily a specific URL, but could be a combination of factors, including:

Hidden tracking pixels: Embedded 1x1 pixel images that redirect through multiple domains to track email opens. If these intermediate domains, like a Heroku burner URL (e.g., meritable-pachyderma-bbbd6478ce61.herokuapp.com), are associated with deceptive behavior or B2B spamware (like Apollo), Mimecast might block the email. You can read more about why images are blocked in emails.
Malicious content detection: The email's HTML structure or embedded content (like certain Google user content URLs) could be triggering Mimecast's anti-virus signatures, even if they aren't directly malicious. This is distinct from a URL reputation block and suggests a deeper content-level issue.
Spamware association: If the email originates from a system known for high volumes of unsolicited mail, Mimecast might block it regardless of the specific links. This means the sender's own domain or IP address might be blocklisted or blacklisted due to association with a disreputable sending platform, rather than the Calendly link itself.

It's important to remember that Mimecast's filters also work outbound. If your organization uses Mimecast, your own outbound emails could be blocked by your organization's own Mimecast policies before they even leave your network if they contain elements deemed risky, reinforcing the need for thorough testing of your outbound email content.

Strategies for overcoming Mimecast URL blocking

When facing Mimecast blocks (or blacklists), there are several strategies you can employ to improve your email deliverability:

Use a custom redirect domain: Instead of directly embedding a Calendly link, use a custom domain for redirects. This works similarly to how ESPs (Email Service Providers) use their own tracking domains. By using a domain you control, you can build a separate, positive reputation for your links, reducing the chance of them being flagged due to the general reputation of services like Calendly or Bitly. Consider using your own website’s domain for this purpose. You can explore how click tracking links get blocked.
Analyze bounce messages: The full bounce message from Mimecast often contains a specific code and a link to their support documentation, providing more detail than a generic 554 error. This information is crucial for pinpointing whether the block is due to a content signature, a specific URL, or a policy configured by the recipient’s organization. You can look up Mimecast SMTP error codes for more context.
Test email content: Send test emails with and without the Calendly link, and with or without any tracking pixels or marketing automation spamware. This A/B testing can help isolate the exact element triggering the Mimecast block. You can also use an email deliverability tester to diagnose potential issues.
Contact the recipient: If the recipient is a Mimecast customer, they might be able to adjust their specific policies to permit your emails. Mimecast allows individual customers to block at the SMTP transaction level, meaning a recipient's internal settings could be the cause.
Utilize Mimecast's sender feedback form: If you are not a Mimecast customer but are experiencing rejections, your IT department can submit a request for review via the Mimecast Sender Feedback form. This can help Mimecast understand and potentially adjust their filtering for your specific sending patterns.

Conclusion

Ensuring your emails reach the inbox requires diligence and a deep understanding of how email security solutions operate. While

Calendly links can sometimes face issues with

Mimecast, the solution often lies in scrutinizing your overall email content, the presence of tracking elements, and the general reputation of all embedded URLs. By being proactive and testing your emails, you can identify and mitigate issues before they impact your deliverability.

Views from the trenches

Best practices

Always include a plain-text version of your email to improve deliverability.

Segment your audience and personalize content to reduce spam complaints.

Regularly clean your email lists to remove inactive or invalid addresses.

Use a custom redirect domain for all external links, especially for scheduling tools.

Monitor your domain reputation diligently to catch issues early.

Common pitfalls

Assuming a legitimate service like Calendly won't be flagged by filters.

Ignoring bounce messages; they provide crucial diagnostic information.

Sending cold outreach without proper consent, increasing spam complaints.

Using generic shortener domains that have a poor reputation.

Not testing email content thoroughly across different email clients and security filters.

Expert tips

The specific error code and link in a Mimecast bounce message are vital for diagnosis.

An email being rejected due to 'security policies' can be due to a general anti-virus signature, not just URLs.

Hidden tracking pixels, especially those redirecting through suspicious domains, are often flagged.

Mimecast's filters can block both inbound and outbound emails if policies are triggered.

If your salesperson is using a sales engagement platform, test combinations of links and platform usage to pinpoint the issue.

Marketer view

Marketer from Email Geeks says they were seeing emails blocked due to Calendly links and hypothesized it was related to Calendly's history of spam issues.

June 5, 2024 - Email Geeks

Marketer view

Marketer from Email Geeks suggests that Calendly blocking is similar to Bitly blocking, due to misuse by 'goobers' for cold lead emails, and suggests using custom redirects.

June 5, 2024 - Email Geeks