My website allows users to send a testimonial referral to their friends via email. These referrals are basically an invite to read a specific testimonial on our website, along with the userís personal message. Some of my users really make use of this feature, often sending several different referrals to one specific recipient. These get queued in the database and are sent out every 30 minutes. Would an ISP who receives 6-8 emails from my site, all at once, consider them to be spam emails? If so, does anyone have a better idea on how I could go about sending referrals so as to not set off red flags?

Thank you!