Problem Description
Someone attempts to send an email to an email address hosted with us and, rather than you receiving the message, the sender receives a bounce indicating simply "5.7.1 Command Rejected"
Problem Resolution
The 5.7.1 Command Rejected bounce message indicates one of the following problems and resolutions. The first problem, SPF failure, is most common (95% of cases with this error), be sure to check this first.
1) SPF Failure
See here to learn about what SPF failure means and how the sender needs to change their DNS records to fix this: https://clients.websavers.ca/whmcs/knowledgebase/22/Email-is-bounced-with-an-error-about-DomainKeys-or-SPF.html
2) Greylisting
The email has been blocked by our greylisting filter. To resolve issues with greylisting please see our guide to disable greylisting for your domain. The guide will walk you through the process of troubleshooting and resolving email greylisting issues.
3) RBLs: Realtime BlockLists
The sender's mail server is on a mail blacklist. We utilize these lists to limit the amount of spam being received. The sender will need to contact their email administrator / provider to have the situation resolved.
