SMTP Settings to enable other app (Acronis TI) to send email via M365

I work mainly with M365 Enterprise but hopefully at least some of the following is also applicable to M365 Business.

If you are sending only to email addresses in your domain and if your Internet service provider (ISP) does not block outgoing port 25 (most business-class ISPs allow it by default or upon request), then you should be able to use the same SMTP server that's configured in your MX record. You would use port 25 and no authentication. It would be like any other email server on the Internet except for the following accomodations:

• If the From address also uses your domain, you will need to do one or more of the folllowing:
  • Add an exception to allow spoofing that address from your IP address.
  • If M365 Business allows it, configure a connector for it in Exchange Online, which you probably want to do anyway to ensure TLS is enforced. To prevent others from using it to spam or phish you, limit connection to it by, for exqmple, allowing only your IP address.
  • Create a mail flow rule to have those emails bypass spam filtering. I recommend multiple conditions such as the sender's From address, a unique header that Acronis TI might include in the email, or the hostname of the sending conputer if it's included in the headers. If you use the Outlook app for your M365 email, the Outlook add-in Message Header Analyzer by Stephen Griffin makes it easy to view the headers. You can also see the Internet headers by look8ng at the Properties of the email.
• If the From address does not use your domain, you will likely need to have it bypass spam filtering.

If you plan on configuring other apps or devices in the future to send emails to yourself, you may want to set up a local SMTP relay to relay emails to M365. Allow it to send emails to the Internet/M365 but otherwise make sure to not allow any incoming connections to it from the Internet; you do not want to accidentally set up a public open relay. Low volume SMTP relaying is very light on resources so the server can be set up using an old computer or in a virtual machine. You can use Windows Server and IIS's SMTP server or, if licensing costs are an issue, use Linux with an open source email server.