Why does my 3rd party SMTP relay flag or send emails to spam? #M365AMA

In this episode, the #M365AMA panel discusses the following community question:

“So we have a customer who uses a 3rd party SMTP relay to send emails from their Cloud based software. At some point 2 days emails started getting Flagged as Spam or phishing and going to Quarantine. No changes were made to anything. But it is driving me nut. Then it seems to work its self out after 11am or so. Seems to start around 11pm at night. Any idea where I should look. The Spam Policy has no blocks on any of these Domains or external users. Nor does the Connection filter. The SPF, Dmarc and Dkim all checks out as well. Our SPF has the Domain for the customers and the Relay. But about to quit IT and start doing Quilting.”

Check out the discussion here:

 

Participating in this discussion were:

• Christian Buckley @buckleyplanet 
• Joy Apple, MVP @JoyOfSharePoint
• Hal Hostetler, MVP  @TVWizard

Some relevant notes/links shared by the team:

• Emails sent through a third-party SMTP relay can end up in spam for several reasons, including:
  • IP blocklisting
  • Unauthenticated email address
  • Sender reputation / user behavior
• Great overview of this topic by Sendgrid: SMTP Relay Service: What Is SMTP Relay and How Does It Work? [https://sendgrid.com/en-us/blog/smtp-relay-service-basics]
• Should You Use a 3rd-Party SMTP Provider for Email Marketing? via Interspire [https://www.interspire.com/should-you-use-3rd-party-smtp-provider-for-email-marketing/]
• Are Your Outbound Emails Going to Spam? Find and Fix Now! via RevBoss [https://revboss.com/blog/are-your-outbound-emails-going-to-spam-find-and-fix-now]