Forum Discussion
SMTP Relay in Exchange Online
Currently we run Exchnage 2019 hybrid. We have not moved our MX records to Exchange Online yet for specific reasons so all mail flow is through Exchange 2019. That said, nearly all user/shared mailbo...
https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365#option-3-configure-a-connector-to-send-emails-using-microsoft-365-or-office-365-smtp-relay
This has methods you can use, if its low volume, and you can use port 25 on your ISP, this method would work for your non-authenticated.
Keep in mind that Microsoft doesn't want people using Exchange Online for bulk mailing. If you're going to bulk mail transactional emails, you will need to look into a different product (Sendgrid, AWS SES, Mailgun, Azure Communication Services, etc.). There are some hard limits coming next year that will impact bulk mailing with Exchange Online.