On Mimecast: Why your website emails soft bounce, even with SPF, DKIM, and Brevo set up

If your website emails are disappearing inside corporate inboxes – even with SPF, DKIM, and Brevo set up, Mimecast might be the culprit. This article breaks down why those emails soft bounce, what Mimecast really looks for, and exactly how to fix it.

On Mimecast blocks: Why emails that look fine still fail

You have set up your website emails correctly. SPF, DKIM, and DMARC records are in place. You are using trusted tools like Brevo (formerly Sendinblue), WP Mail SMTP, and WordPress. Emails land in Gmail and Outlook without a problem. But inside organisations using Mimecast email security, your messages are soft bounced, greylisted, or worse – blocked completely.

If that sounds familiar, you are not alone. Mimecast enforces a much higher standard for email authenticity. It does not just check your DNS records. It interrogates your server identity, your email headers, and your platform behaviour. And if there is even a hint of inconsistency, it stops your message cold.

How did I do, worth a Google Review?

On Real-world setups: how WordPress and Brevo exposed the issue

One of our clients faced exactly this issue:

• WordPress website with cloud hosting 
• Contact Form 7 for user enquiries
• WP Mail SMTP for outbound delivery
• Brevo (Sendinblue) as the SMTP provider
• Full SPF, DKIM, and DMARC compliance 

Emails were arriving externally but consistently greylisted or soft bounced internally by Mimecast.

On Mimecast security: how deeper checks reveal delivery issues

Mimecast doesn’t just validate SPF, DKIM, and DMARC. It applies header-based anti-spoofing checks that scrutinise:

• The Return-Path domain
• The SMTP HELO/EHLO hostname
• The visible From address 

If these elements are inconsistent – or if your sending IP isn’t explicitly trusted – Mimecast may delay the message, soft bounce it, or even reject it.

Greylisting, soft bouncing (often 451 errors), and header anomalies are common symptoms when Mimecast’s policies are triggered.

On DNS: why SPF, DKIM, and DMARC alone are not enough

SPF, DKIM, and DMARC are foundational. But they mainly verify domain ownership, not server identity alignment.

Mimecast email security expects:

• Alignment between SMTP server introduction and From address
• Consistent Return-Path and visible sender
• Trusted IP addresses, verified beyond just DNS records 

Without these elements, even a “perfect” DNS setup fails under deeper inspection.

On Fixing it: how bypassing greylisting and anti-spoofing solved the problem

Resolution came through Mimecast policy adjustments:

• Bypass greylisting for Brevo’s IP address ranges
• Bypass anti-spoofing for emails from the client’s domain via Brevo 

After these adjustments, emails started flowing correctly without delay or soft bounces.

On Deliverability: your Mimecast checklist for website emails

If you want your WordPress website emails to be delivered reliably through Mimecast, here is what you must check:

• Correct SPF, DKIM, and DMARC authentication
• Ensure Return-Path and From address use the same domain
• Confirm your SMTP HELO/EHLO hostname matches your brand domain
• Add your SMTP provider’s IP ranges (e.g., Brevo) to Mimecast trusted senders
• Implement Bypass greylisting and Bypass anti-spoofing policies for your sender IPs
• Regularly test full email headers for consistency and policy compliance 

How did I do, worth a Google Review?

On Compliance: why Mimecast standards define brand trust

Email delivery isn’t just a technical detail. It is a brand experience. If a form submission, enquiry, or transaction confirmation doesn’t reach its destination, your business risks losing trust.

In the age of enterprise-grade security platforms like Mimecast, standard DNS setups are no longer enough. Server identity, policy trust, and header alignment are critical.

At HeightOn, we specialise in closing these invisible gaps – from your website platform to your customer’s inbox. If you are serious about better digital performance, we are ready to help you build a solution that works.