Welcome to Microsoft 365 or Office 365

It is best to set up your SMTP with a third-party authentication password instead of a regular mailbox. This will help to ensure emails are sent and not flagged as spam. 

Read Microsoft's Learning Information on how to manage this here: 

Option 1: Authenticate your device or application directly with a Microsoft 365 or Office 365 mailbox, and send mail using SMTP AUTH client submission