Thank you for your question.
Verizon has the right to label your email as spam. Your friend may need to switch email providers in order to receive the email with an attachment.
In regard to your legal point of having the right to communicate, this right is against the government, not a private corporation. Further, the right to communicate is based on the right to directly speak to another person or use the public mails. An internet service provider's email system is a private system to which this right does not apply.
In regard to equal protection under the law, again this is a right which applies to the government, not to private corporations.
In regard to tapping a phone, this is not the same circumstance. This email is coming through and triggering a computer program which identifies it as spam for whatever reason. They are not opening up your email and reading it, otherwise this would be illegal interception of a private electronic communication.
Private communications, such as this, are a matter of contract
law. So if you are sending it to your friend's email account and her internet service provider will not let it through, then she needs to contact them as she is the customer.