Spam complaint rate

Gmail shows it in a free tool called Postmaster Tools. Gmail's guidelines ask all senders to keep it below 0.3%, and suggest staying below 0.1%. That is 1 complaint in 1,000 emails.

Most complaints come from people who did not expect your email. Send only to people who fit your ideal customer profile, write personal messages, and make it easy to opt out. For cold email, use a separate domain to protect your main email deliverability, and warm it up before you send at volume.