Why is Outlook displaying phishing warnings on emails sent from my CRM through Sendgrid, and how can I fix it?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 Jun 2025
Updated 16 Aug 2025
10 min read
When sending emails from your CRM through SendGrid, encountering phishing warnings in Outlook can be a frustrating experience. You're trying to communicate legitimately with your contacts, yet Microsoft Outlook flags your messages with a stern warning like, "This email was sent from outside your organization, yet is displaying the name of someone from your organization. This often happens in phishing attempts. Please only interact with this email if you know its source and that the content is safe." This issue not only hinders your communication but also damages your sender reputation and can lead to emails landing in the spam folder, junk mail, or even a blocklist.
This problem often stems from how email authentication protocols are configured, or sometimes misconfigured, when a third-party email service provider (ESP) like SendGrid sends on behalf of your domain. Outlook, specifically Microsoft 365, has robust anti-phishing mechanisms designed to protect its users from business email compromise (BEC) and spoofing attacks. Understanding these mechanisms and ensuring your email setup aligns with best practices is crucial for overcoming these deliverability hurdles.
These warnings can be particularly challenging because they directly impact user trust and engagement. Recipients seeing such alerts may become hesitant to open future emails from your domain, even if they are legitimate. This can undermine your marketing efforts, transactional communications, and overall business operations.
My goal here is to demystify why these warnings appear and provide actionable steps to resolve them. It's about ensuring your legitimate emails are recognized as such, building trust with recipients, and maintaining a healthy email ecosystem. We'll explore the technical aspects, from email authentication standards to sender reputation, and how they collectively influence Outlook's filtering decisions.
Understanding outlook's phishing warnings
Outlook's phishing warnings are a security feature designed to alert users about potentially malicious emails. These warnings are particularly common when the "From" address (the sender's name and domain) appears to be internal to an organization, but the email originates from an external source or IP address. For instance, if your CRM sends an email that looks like it's from "sales@yourcompany.com" but is technically sent by SendGrid's servers, Outlook might see this as suspicious.
The primary reason for such warnings is often a mismatch in how the email is authenticated versus how it appears to the recipient. Outlook's filters are vigilant about detecting spoofing, where malicious actors try to impersonate legitimate senders. If your email lacks proper authentication, or if the authentication aligns with the sending service (SendGrid) rather than your domain, Outlook's "spoof intelligence" will flag it. Microsoft provides guidance on dealing with phishing and suspicious behavior in Outlook, emphasizing the importance of robust security measures.
Essentially, Outlook wants to verify that the email sender is truly who they claim to be. When an email purporting to be from your domain arrives, but the underlying authentication details point elsewhere or are weak, it raises a red flag. This protective measure, while sometimes inconvenient for legitimate senders, is vital for safeguarding users against prevalent phishing scams. For more insights on this, you can refer to Microsoft's support documentation on why emails are marked as spam.
Authentication: the core of trust
Email authentication protocols are the bedrock of email deliverability and are critical to preventing phishing warnings. These protocols, including SPF, DKIM, and DMARC, tell recipient servers whether an email is legitimate and authorized to be sent from a particular domain. When you send emails through a third-party service like SendGrid, it's paramount that these protocols are correctly configured for your domain.
SPF (Sender Policy Framework): This record specifies which mail servers are authorized to send email on behalf of your domain. If SendGrid is sending your emails, your SPF record must include SendGrid's sending IP addresses or their designated include mechanism, such as include:sendgrid.net. If your SPF record is incomplete, Outlook will treat emails as suspicious because they appear to come from an unauthorized source.
DKIM (DomainKeys Identified Mail): DKIM adds a digital signature to your emails, allowing the recipient server to verify that the email content hasn't been tampered with and that it originates from an authorized domain. When using SendGrid, you should configure DKIM to sign emails with your domain (d=yourdomain.com), not d=sendgrid.net. This is a common pitfall that triggers phishing warnings; if the DKIM signature shows SendGrid's domain, Outlook perceives it as an external entity trying to send on your behalf, especially if the "From" address is your internal domain. We have a guide on how to fix common DMARC issues that covers this in more detail.
DMARC (Domain-based Message Authentication, Reporting, and Conformance): DMARC builds upon SPF and DKIM, allowing you to tell receiving servers what to do with emails that fail authentication (e.g., quarantine, reject, or none). A strong DMARC policy (p=quarantine or p=reject) ensures that your domain is protected from unauthorized use and significantly improves trust with mail providers like Outlook. Setting up DMARC monitoring can provide valuable insights into your email authentication performance.
Practical solutions to fix phishing warnings
Addressing Outlook's phishing warnings requires a systematic approach, primarily focusing on robust email authentication and maintaining a good sender reputation. Here are the key steps to take.
Common issues
Authentication mismatch: Emails failing SPF or DKIM alignment, often due to SendGrid authenticating with its own domain instead of yours.
Weak DMARC policy: A DMARC policy set to p=none provides no instruction to Outlook on how to handle unauthenticated mail.
Poor sender reputation: High spam rates or frequent blocklist appearances can trigger warnings, regardless of authentication.
Content flags: Email content, links, or attachments might resemble known phishing patterns.
Solutions
Configure custom DKIM: Ensure SendGrid is configured to sign emails with your domain (d=yourdomain.com). This is often called 'whitelabeling' or 'custom sender authentication' in SendGrid's settings.
Implement strong DMARC: Gradually move your DMARC policy from p=none to p=quarantine or p=reject after careful monitoring.
Monitor sender reputation: Regularly check your domain and IP reputation using Google Postmaster Tools V2 and address any issues promptly. Also monitor for being listed on any email blacklist or blocklist.
Review email content: Avoid suspicious keywords, overly generic links, or deceptive formatting that might resemble phishing. For more help you can read our guide on why your emails are going to spam.
You've likely configured your SPF record to include sendgrid.net, which is a good start. However, SPF alone might not be enough to prevent these warnings, especially with Outlook's stricter filtering. The key often lies in DKIM alignment and DMARC policy enforcement.
I often see scenarios where the email's "From" header (what the user sees) is user@yourdomain.com, but the underlying DKIM signature's d= tag or the Return-Path (used for SPF checks) points to sendgrid.net or a subdomain of it. This creates a misalignment that Outlook interprets as a spoofing attempt. To fix this, you need to ensure that SendGrid is configured to sign your outgoing emails with your domain (yourdomain.com) for DKIM. This means adding specific DNS records provided by SendGrid to your domain's DNS.
SendGrid DKIM DNS recordsDNS
s1._domainkey.yourdomain.com. IN CNAME s1.domainkey.sendgrid.net.
s2._domainkey.yourdomain.com. IN CNAME s2.domainkey.sendgrid.net.
Once these records are in place and propagated, emails sent via SendGrid from your CRM should have DKIM signatures aligned with yourdomain.com, which helps Outlook recognize them as legitimate. This is often the critical step missed in these situations. Additionally, ensure your DMARC record is set up to p=quarantine or p=reject once you are confident that all your legitimate sending sources are properly authenticated. You can use our free DMARC record generator tool if you need help getting started.
Beyond technical fixes: content and user behavior
While technical configurations like SPF, DKIM, and DMARC are fundamental, other factors also influence how Outlook perceives your emails. The content of your emails and the behavior of your recipients play significant roles in determining whether your messages trigger phishing warnings or land in the inbox.
First, consider the email content itself. Generic subject lines, unusual sender names (even if technically correct), or links that don't clearly point to your domain can raise suspicion. Phishing attacks often rely on social engineering, using urgent language or unexpected attachments. Even if your emails are legitimate, if they inadvertently mimic these patterns, they might be flagged.
It's essential to regularly review your email templates for anything that could be misinterpreted as suspicious. Pay close attention to calls to action, embedded links, and the overall tone. Clarity and transparency in your messaging can go a long way in building trust with both your recipients and email providers. Ensure that your CRM's email templates are designed with deliverability in mind, avoiding any elements that could inadvertently trigger spam or phishing filters.
Recipient engagement is also crucial. If your emails consistently go unread, are deleted without opening, or worse, marked as spam by recipients, this negatively impacts your sender reputation over time. Outlook's filters learn from user behavior. Encourage recipients to add your sending address to their safe sender list or contacts. Providing clear unsubscribe options and sending only to engaged audiences can drastically improve your standing with mail providers and reduce the likelihood of your emails being caught by a phishing blocklist or general spam filters.
Views from the trenches
Best practices
Prioritise strong email authentication for your sending domain.
Configure SendGrid's custom DKIM signing using your own domain name.
Implement a DMARC policy to quarantine or reject unauthenticated email.
Regularly monitor your domain and IP reputation using available tools.
Design email content carefully to avoid resembling phishing patterns.
Common pitfalls
Relying solely on SPF without proper DKIM alignment and DMARC enforcement.
Using SendGrid's default DKIM signature instead of your own domain's custom one.
Having a DMARC policy set to 'p=none' indefinitely, offering no protection.
Neglecting sender reputation metrics and ignoring user engagement signals.
Sending emails that contain suspicious keywords or overly generic links.
Expert tips
Proactively check Outlook's specific warnings for clues on their policy triggers.
Verify that your CRM is properly configured to use the authenticated SendGrid setup.
Gradually move your DMARC policy to enforcement after thorough testing and monitoring.
Advise internal IT admins on necessary Microsoft 365 security portal adjustments for exceptions.
Consistent monitoring and iterative improvements are key to long-term deliverability success.
Marketer view
Marketer from Email Geeks says: We've been seeing Outlook warnings on emails that appear to be from within our organization but originate externally, flagging them as potential phishing attempts.
2021-09-01 - Email Geeks
Marketer view
Marketer from Email Geeks says: This type of warning often indicates a Business Email Compromise (BEC) spam pattern, so it's best to consult with your IT administrators.
2021-09-01 - Email Geeks
How to secure your email sending
Resolving Outlook's phishing warnings for emails sent via CRM through SendGrid boils down to establishing clear, undeniable trust. This trust is primarily built through robust email authentication protocols: SPF, DKIM, and DMARC. Ensuring that your emails are not only technically allowed to be sent by SendGrid but are also explicitly signed by your domain is the most critical step.
Beyond the technical setup, continuous vigilance over your sender reputation and thoughtful email content creation are equally important. By implementing custom DKIM signing with your domain, enforcing a strong DMARC policy, and regularly monitoring your deliverability, you can significantly reduce the likelihood of your legitimate emails being flagged. This approach ensures your communications reach their intended recipients, fostering better engagement and protecting your brand's integrity.
Microsoft Outlook flags your messages with a stern warning like, "This email was sent from outside your organization, yet is displaying the name of someone from your organization. This often happens in phishing attempts. Please only interact with this email if you know its source and that the content is safe." This issue not only hinders your communication but also damages your sender reputation and can lead to emails landing in the spam folder, junk mail, or even a blocklist.
SendGrid is configured to sign emails with your domain (d=yourdomain.com). This is often called 'whitelabeling' or 'custom sender authentication' in SendGrid's settings.
