SMTP Error 451 4.7.500 Server busy. Please try again later.

Symptoms

When sending email to a domain hosted on an Office 365 or Microsoft 365 mail server, you encounter a bounce message similar to the following.

SMTP error from remote mail server after end of data: 451 4.7.500 Server busy. Please try again later from [203.0.113.2]. (S77719) [PU1APC01FT043.eop-APC01.prod.protection.outlook.com]

Description

The error occurs when the recipient mail server's Exchange Online Protection (EOP) service detects a significant change in the sending mail server's previously established sending patterns.

Please note that the issue is not related to the cPanel server itself, but rather stems from the remote host.

Workaround

The remote mail server's administrator must set up a connector between their server and your server.

Additional resources

Set up connectors to route mail between Microsoft 365 or Office 365 and your own email servers