The recipients email provider or security applications changed the subject line that was originally sent. If a recipient has Antivirus or Anti-malware on their computer set to check incoming mail this could add in a note to the subject line and look to rename the subject line as (Spam) or other verbiage such as "remove from future mailings" - if a rule has been set by their malware to indicate possible threatening emails. This is generally a tag that is set in the recipient's malware program.
The recipients can check their security settings set on their antivirus or malware to see if they can change those settings.
Here is a link to McAfee (a common antivirus program) that discusses this setting.
If you look on page 1 of the "email filtering" it talks about a tag where email subjects are changed due to the spam level of the email.
You could also try and have the recipients that see this subject line change whitelist the IP address mass emails would originate from allowing all emails that come from you to pass security settings. Keep in mind that depending on what software they are using their antivirus software may over ride that.
Here are the IP addresses that should be white listed by the recipient in their email program:
eTapestry's IP Addresses: