first thing first, you messaging. check if it's weak or generic. using spammy words or the same templates over and over again can also be one of the factor (since it'll be flagged by gmail immediately) so make sure to check your words and change it a bit after a while. check your inbox health/deliverability. if it's <80 then you'd prob need to create more inboxes. always do warm up and batch your mail (don;t send it all at the same time)