How to avoid email content filtering

Dave Hilditch asked:

I have an online tool that works via email.

Some emails are not getting through and I discovered the “550 5.7.1” Message Rejected due to content restrictions error.

Things I’ve discovered that can cause messages to be flagged as suspect include:

  1. Hyphens in the domain name
  2. Hyphens in the sender name

I’ve not discovered much more info however and unfortunately neither of these is the cause in my case.

Here are the contents of a sample email:

Subject: Improved Employees Soft Skills Link
From: Improved Employees ([email protected])
To: David Hilditch ([email protected])
Hello David Hilditch,

Your Improved Employees SoftSkills account has been created and is ready to use.

Please send the following link to all job applicants to assess their soft skills:

Soft Skills Test
Thank you. Improved Employees p.s. You can also include the link above in job adverts to save you having to email each candidate. p.p.s. You'll find a PDF quick start guide attached.

I am trying to get in touch with Cloudmark as it seems that they are the ones at the backend who are marking my mails as spam, but I tried to find a resource explaining what else can be done to the message to make it more likely it doesn’t get marked as spam.

I have identified the following possibilities – does anyone know which of these will make a difference and are there any others that would reduce the likelihood of my emails being marked as spam?

  1. Use my own email address as REPLY-TO?
  2. Use my own name as FROM NAME?
  3. Don’t attach a PDF (I thought these were safe?)
  4. Include my name at the bottom of the email?
  5. Include a message about why they’re receiving this email?
  6. Include an ‘unsubscribe’ link?
  7. Don’t include the hyperlink with the parameterised consultantid. (this would be very annoying if I had to do this)
  8. Change ?consultantid=10867 to #10867?

Any help in understanding this content restriction filter very much appreciated as this is legitimate use in my pre-sales pipeline and ultimately actual use of my product and I really don’t need the headache of emails not making it to their destination. (I have the SPF record created to mark my server as an authentic sender)

If you want to generate a sample email, you can create an account at the following link which will send the email described above including the attachment:

My answer:

“Content restrictions” suggests that something in the email content is the problem.

The prime suspect, of course, is the file attachment. You would be well advised to not send these as attachments, but to provide a download link to the file in your web site instead.

View the full question and any other answers on Server Fault.

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