The average open rates for emails are around 20%. If you encountered drastically high email open rates, it may be caused by spam filters opening your emails before they reached the recipients' inbox.
In order to protect their users from malicious content, inbox providers like Gmail and Outlook use spam filters to open and check content from "untrusted senders". This normally happens when you just started sending emails from a domain without much sender reputation.
Identify Spam Filters Opening Emails
The simplest way to identify if an email is opened by spam filters is to look at the time difference between email delivered and opened. If the time difference is seconds apart, you can assume that email is opened by spam filter instead of actual user.
By default, OnVoard will try to detect opens/clicks from spam filters. We do this by automatically exclude opens/clicks that happen within 5 seconds after the email was delivered.
OnVoard is not using user agent to detect opens from spam filters since Gmail serves pixel image to users via their proxy server. Thus, the user agent that we see on our end from Gmail users are actually from Gmail servers.
Avoid Spam Filters
To avoid triggering spam filters, you need to slowly build up your sender reputation by sending emails only to users who are expecting them. Things like email engagement (bounce/spam rates) will affect your reputation. See Who Can You Send To.