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When messages are sent or received between two email servers or Mail Transfer Agents (MTAs), the communication uses a series of numeric SMTP codes. These codes are always in pairs, which means both servers transmit the codes until either the conversation is successful, or fails.   There are two main code types for dropped or failed SMTP

What You'll Need Walkthrough Updating the SPF Record for your Domain(s) Configuring Outbound Routing Adding the Office 365 Tenant Domain as an Internal Domain Verifying Your Configuration This step must be completed to route emails outbound from Office 365 to Mimecast. We recommend completing this step at least three