Why This Business’s Emails Were Suddenly Being Rejected, A Case Study From PC Onsite IT Solutions

Email is one of the most important communication tools for any business. So when emails start bouncing or being marked as suspicious, it can interrupt sales, delay urgent communication, and frustrate everyone involved.

Recently, a customer reached out to us because their emails were suddenly being rejected by multiple recipients, even though nothing had changed on their side. No new devices, no password resets, no settings modified. Everything looked normal.

But something wasn’t right. This is the kind of problem we diagnose regularly at PC Onsite, so we began a full investigation.

The Issue: Emails Being Rejected With No Clear Cause

The customer was receiving bounce-backs and rejection messages such as:

• “Email marked as suspicious”

• “Message rejected due to domain reputation”

• “This message resembles spam or phishing”

  
  

At first glance, their Microsoft 365 environment was healthy:

• Mailbox functioning normally

• DNS records are correctly set

• No authentication failures

• No evidence of compromised email accounts

  
  

So why were recipients’ mail servers blocking their messages?

The Discovery: A Hacked Website Caused Domain Reputation Problems

During our analysis, we turned our attention to the customer’s website, which uses the same domain name as their email.

That’s where we found the problem: the website had been compromised (hacked),

This is something most business owners don’t realise.

Your website’s security directly affects your email deliverability.

Modern spam filters and security systems check everything linked to your email—including your domain, website, and any hyperlinks in your signature.

Because the website contained malicious or unauthorised code, email providers began to view anything from that domain as unsafe.So even normal business emails were flagged as suspicious or rejected outright.

Why This Happens: How Website Hacks Affect Email Reputation

When a website is hacked, security systems may detect:

• Malware injections

• Redirects to unsafe sites

• Suspicious scripts

• Unexpected changes in site behaviour

• Unauthorised files

  
  

Once this happens, multiple systems across the internet may mark the domain as unsafe. This includes:

• Microsoft

• Google

• SpamHaus

• Security vendors

• Email firewalls

• Anti-phishing engines

So when you send an email with a signature that links to your website, the receiving server sees:

“This domain is unsafe → Block or reject the message.”

That’s exactly what happened to our customer.

How We Fixed It: PC Onsite’s Remediation Process

Once we identified the root cause, we acted quickly to protect the customer’s brand and restore their communication.

1. Website Cleanup & Security Repair

• Removed all malicious and unauthorised code

• Cleaned suspicious files and directories

• Updated CMS, plugins and themes

• Reset passwords and secured admin accounts

• Enabled protective measures to prevent reinfection

  
  

2. Email Security Verification

We ensured all authentication records were correct and active:

• SPF

• DKIM

• DMARC

  
  

These are critical for email trust and deliverability.

3. Domain Reputation Recovery

To help restore normal email flow, we:

• Requested a re-scan from major security providers

• Submitted delisting requests where needed

• Monitored email deliverability during the recovery

  
  

4. Follow-Up Testing

We sent controlled test emails to confirm:

• No more rejection messages

• No spam flagging

• Normal delivery times

  
  

Everything returned to normal after the website was cleaned and trust was restored.

The Outcome: Email Delivery Restored and Website Secured

Once the website was repaired and security systems re-evaluated the domain, the customer’s email delivery quickly improved.

Results included:

• No more rejected emails

• No more spam warnings

• Stable domain reputation recovery

• Improved website security and performance

• Peace of mind for the business owner

  
  

This is a great example of how one hidden issue on a website can disrupt your entire business communication.

Key Takeaways for Business Owners

Here are the most important lessons from this case:

1. Your website and email reputation are connected.

If your website is hacked, your emails can be blocked—even if your mail server is fine.

2. Deliverability issues usually have deeper causes.

Email settings alone rarely cause widespread rejection.

3. Quick action matters.

The longer a domain remains flagged, the harder and slower the recovery.

4. Regular security maintenance is essential.

Hackers target outdated plugins, weak passwords, and unmonitored websites.

Need Help? PC Onsite IT Solutions Is Here for You

If you ever notice:

• Emails bouncing

• Delivery issues

• Customers are not receiving your messages

• A strange change in your website

• Unexpected redirects or slow loading

  
  

Don’t ignore it. These are often early signs of domain or website compromise.

At PC Onsite IT Solutions, we specialise in:

• Website security and recovery

• Microsoft 365 and Google Workspace troubleshooting

• Email deliverability analysis

• Cybersecurity audits

• Domain and DNS configuration

• Ongoing IT support and monitoring

  
  

Want us to check your website or email system? Contact us.

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