When you host your email on the Exchange Online (EXO) platform part of Office365 you can implement several security measures to make sure that email send from your domain gets delivered to the mailbox of the recipient.

The most known solution for this is by implementing a Sender Policy Framework (SPF) DNS record. By creating a SPF DNS record in your DNS you provide receiving email servers the option to check if the originating IP of the email is also the authorized email server for the domain. If not the email can be considered suspicious and the email system from that point forward can decide to threat the email as spam, phishing and so forth. 

If you decide to make the nameservers of Microsoft authoritative, which allows you to manage your DNS settings from the Office administration portal, the SPF record needed can automatically be enabled for you.

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