Should I use a mail relay server or direct sending for devices with Office 365?

While you can technically use Office 365 for this, it ends up being less flexible and requires more configuration steps. It’ll likely be easier and more adaptable in the long run to set up an internal relay or go with something like SMTP2GO. Less hassle for sure!

If you plan to use direct send, make sure to add the IP that will appear as the sender to your SPF record in DNS. Note that direct send only sends to recipients within your organization, but I’ve got it running smoothly for some scanners at my company. Larger organizations might face issues, but it should work fine for smaller setups. Also, consider if a DNS redirect to Microsoft's Direct Send server could help you avoid reprogramming every device. I haven't tried it, but it might be worth checking out!

For devices that don't support modern authentication, going directly through Office 365 could be tricky. It might be a lot more effective to use a cloud SMTP provider instead.

We recently did this too! We set up a Postfix server for anonymous SMTP that uses DNS to route messages correctly. With a connector in Office 365 to classify them as internal, it worked really well, provided you also set up SPF, DKIM, and DMARC correctly.

I've had great luck with SMTP2GO; it's one of the easiest options out there! Just setup and forget about it!

Setting up a Postfix mail relay is a solid option. You have better control over SMTP authentication and can monitor it easily. If you're leaning towards an all-Microsoft solution, then the mail relay server approach on IIS should work well, given you use the same IP as your old Exchange server.