The ability to send email from your cart and have it be received by customers is critically important. Assuming you are already following best practices and not sending unwanted email, the best way to safeguard your sending reputation is to not send bulk emails using the same mechanism you use to send transactional emails.
I have introduced a new PR into Zen Cart 1.5.8 (which can be backported to 1.5.7 if needed) to facilitate this. A new set of configuration parameters allows you to identify an email sender (like SMTP2Go, SendGrid, etc.) for bulk email, which is completely separate from your regular email sender. Let me know your thoughts on Configuration settings for non-transactional email, PR#4662.