Mailtrack sends emails through Gmail’s servers because they offer the best deliverability. This means Mailtrack has no control over Gmail’s sending limits and blocking policies.
If Gmail detects any suspicious spam activity, it may decide to block emails from being sent. This is true even if the daily limit has not been reached.
In case of blocking, Gmail does not want to give tips to offenders and sends generic error messages.
Maybe the recipient has flagged your email or your domain as spam in the past. Maybe your emails include some words that increase the spam score. Maybe your email includes a link to a domain with a bad reputation. It's impossible to know the exact reason. Gmail can block mass emails, but let through individual emails to the same recipients.
Gmail is constantly improving its system to detect and block spam. We recommend not trying to circumvent Google's policies. Because what works now may not do in the future. If Google has mistakenly blocked your email sends, you'd need to contact Gmail.