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:


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:


  • You'll probably find it's more to do with a lack of other headers/formatting that are present in most emails sent from normal/interactive email clients. Suggest you compose the same message in your normal MUA and compare the raw message generated with that produced by your software.
    – eggyal
    Jan 17, 2013 at 17:38
  • Thanks - thanks to this I noticed that sometimes I've been failing the SPF test - I modified my code to use the actual IP address of the mail server (rather than and it passes the SPF now - weird... Jan 20, 2013 at 20:25

2 Answers 2


"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.

  • I could remove the PDF attachment from this introductory email, but every email they receive after this is going to have a PDF attached hence the 'test' with the PDF attached to the intro email. Jan 20, 2013 at 20:27

We have issues even with internal emails. Outlook's spam/ phishing filter seems to randomly drop messages with a hyperlink. For automated messages that must get through, we stopped adding active links. We just include as regular text the part that follows http:.

  • Thanks - I've modified the contents of all the emails to remove http:// - still awaiting results on new clients to see the impact. Jan 20, 2013 at 20:26

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .