Mail being sent by CFMAIL blocked by some ISPs

I have written an app for a client which sends emails as part of confirmation. We have noticed that some domains will block the email ie btinternet.com email addresses will block the emails. However, if the client emails from their own email address the customer will receive this email. What could be blocking this email?
We are using CF9

Here is my email code:
<cfmail to="#customer_email#" from="#from_email#" bcc="#blankemail#" subject="#email_subject#" type="html">
<cfmailpart type="text/plain">
<cfoutput>#plain_email_text#</cfoutput>
</cfmailpart>
<cfmailpart type="text/html">
<cfoutput>#html_email_text#</cfoutput>
</cfmailpart>
</cfmail>