If external business partners or clients are complaining that their emails to your business address are bouncing back with "Blocked," "Rejected," or "Delivery Failure" errors, this guide is for you.

We will explain how our spam filtering works, why certain emails get blocked, and what we can do to get your business communications flowing smoothly again.

Can I Whitelist an Entire Domain Across My Whole Company?

Many businesses ask if they can log into their Plesk Control Panel and add a global rule (like whitelisting @importantpartner.com) so that emails from that domain bypass security checks for all of their employee mailboxes at once.

To protect your business from email spoofing (where spammers pretend to send emails from your trusted partners), Plesk does not allow domain-wide whitelisting at the user level.

Allowing whole-domain whitelisting across an entire server creates severe security vulnerabilities. Instead, spam-filtering rules are applied safely using a combination of individual mailbox whitelists and advanced server-wide security layers.

The 4 Reasons External Emails Get Blocked

Our email servers employ four separate defense layers to stop spam and phishing attacks before they reach your inbox. If a legitimate email fails one of these checks, it will bounce back to the sender.

1. The Sender has a Misconfigured SPF Record (Most Common)

Modern email servers use SPF (Sender Policy Framework) to verify that the server sending the email is officially authorized to do so.

• The Problem: If your partner's IT team recently updated their mail system but forgot to update their SPF record, our server will detect a mismatch and reject the email to protect you from spoofing.
   

• The Fix: Your partner's IT team must update their DNS records to ensure they have a valid, passing SPF record.

2. The Sending Server is on a Public Blacklist

Our servers check incoming mail IPs against major public blocklists (like Spamcop or Spamhaus).

• The Problem: If someone else on your partner's mail server sent out bulk spam, that entire server's IP address might get flagged and blacklisted.
   

• The Fix: The sender's IT provider must contact the blacklist operator (e.g., spamcop.net) to request a standard IP delisting.

3. The Email Triggered a "Greylisting" Delay

Our server uses a smart defense tool called Greylisting to stop automated spam bots.

• The Problem: When an unknown mail server contacts us for the first time, our server temporarily defers it. Legitimate mail servers will automatically retry a few minutes later and succeed. Spam bots will not retry. Sometimes, this can cause a slight delay on the very first email sent by a new contact.

4. SpamAssassin Content Filtering

Our local scanner reads incoming emails for spam-like content, excessive links, or bad headers.

• The Fix: You can bypass this check for specific trusted senders by whitelisting them directly inside your personal mailbox:

  1. Log into your Plesk Control Panel.

  2. Go to Mail > Mail Accounts and select your email address.

  3. Click on the Spam Filter tab and click Show Advanced Settings.

  4. Add your partner's specific email or wildcard domain (e.g., *@importantpartner.com) to your local White List and click OK.

Still Blocked? Let Us Investigate the Server Logs!

If your partners are still unable to send you emails and you suspect a server-wide block is in place, our engineering team can look directly into our mail delivery logs to find the exact rule that is blocking them and clear it.

How to Request an Investigation:

To help us find the block, please email us at service@rebel.com and provide as much of the following information as possible:

1. The Sender's Domain or Email Address (e.g., partnerdomain.com).

2. The Approximate Time and Date they attempted to send the email.

3. The Bounce-back Error / .EML File: Ask your contact to export the "Delivery Failure" notification they received as an .eml file and send it to you, then attach it to your ticket with us.

Our team will locate the transaction in the system logs, determine exactly why our firewall or anti-spam filters rejected the message, and take the necessary steps to restore your delivery!

Contact Rebel Support

• Email Us: service@rebel.com (Feel free to email us anytime, and our team will get back to you as soon as possible!)

• Call Us: 1-844-548-2157

Support Availability Summary

Need to speak with a live representative via Phone or Live Chat?

• Live Chat & Phones (Mon-Fri): 8:00 AM – 8:00 PM EST

• Live Chat & Phones (Sat-Sun): 8:00 AM – 5:00 PM EST