The error 451 4.7.500 Server busy indicates that the mail server (Microsoft 365 / Exchange Online) is temporarily rejecting emails due to protection policies, such as greylisting or sending pattern detection.
Reason for the error
This error can occur when the Microsoft Office 365 / Exchange Online servers:
Detect new sending patterns
Identify the source as potentially untrusted
Apply protection mechanisms such as greylisting
As a result:
Emails may be temporarily rejected
Or delayed in their delivery
Error example:
Error description example:
This can impact the reception of emails sent from SMARTFENSE.
Why does this happen?
This behavior is part of Microsoft's security measures to:
Prevent spam
Detect unusual mass sending
Validate the reputation of the email source
It is especially common when:
Sending is initiated from a new source
Sending settings are modified
Mass mailings are performed for the first time
How to fix it?
It is recommended to:
Analyze the mail environment configuration together with the organization's technical team
Review security policies in Microsoft 365
Evaluate the configuration of connectors or transport rules
In particular, it may be necessary to:
Configure a connector in Exchange Online
Adjust filtering policies
Review security settings that may be blocking the sendings
Considerations
This error is temporary and may resolve automatically over time
It is not an error caused by SMARTFENSE
It depends on the configuration and policies of the client's Microsoft environment
💡 Best practices
Conduct test campaigns before mass mailings
Coordinate configurations with the client's IT team
Properly implement whitelist and connector configurations