about email inbox issue

Hi ,

Im email marketer, i'm sending marketing emails to my subscribers, i passed all authentications to my sending domain, last few months we are facing big issues, my mailing servers are reacted as hourly inbox server that is like if i send marketing mails to my clients only 1400 mails are going inbox to my subscribers inbox, once it reached 1.4k emails, other emails are automatically going to spam folder , i can not find the reason for that, and next hour 1400 emails are going to inbox , once i reached 1400 mails and i try to send mails to my clients , mails are going to spam.