I seems exchange server spam filter or any filtering gate like barracuda or anyone which you are using treating these emails as spam or another reason might be you have raid limit in your spam filtering gateway where emails limit is mention like 50 mails per hour or like that other mails will send after raid limit release .
please also find link for help if issue resolve please accept answer
https://learn.microsoft.com/en-us/exchange/mail-flow/queues/message-procedures?view=exchserver-2019