Hello Brett Jhonson,
Thanks for reaching out to the Microsoft Community. We are happy to assist you.
To use SMTP to send emails through Microsoft 365, you need to configure SMTP client submission. Regarding your question about licensing, you need to have a valid license assigned to the account you are using for SMTP client submission. If you don't have a license assigned to the account, you won't be able to use SMTP client submission.
You may refer to the article for detailed information see under Option 1: Authenticate your device or application directly with a Microsoft 365 or Office 365 mailbox, and send mail using SMTP AUTH client submission
As for using a third-party SMTP service provider like SMTPget or SendGrid, it's up to you to decide which option is better. However, using Microsoft 365's SMTP client submission is a more secure and reliable option as it is integrated with Microsoft's security features and compliance standards.
Thank you for your understanding. If you have any questions or need to add more information, please let us know.