Microsoft has two specific services for sending app generated mass mails, which they recommend over their Client SMTP submission:
If you only have internal recipients, you can use HVE, which is easier to setup:
If not, you can use Azure Communication Services:
The normal email infrastructure of M365 is not meant to be used for app generated mass mails. The rate limits can easily be reached, even by a medium Zammad instance. Both methods above support/require Basic Auth.