By default, Google Cloud Platform does not allow SMTP(e-mail) from standard ports from their virtual machine instances. They have a detailed article that talks about ways around this using third-party vendors, which can be found here. This will impact users who are trying to send out documents from applications such as QuickBooks, Sage, etc.
For some of our clients, we've been able to use MailGun's free offering. Although, you need to specify the user addresses that you will be sending mail to in a web interface. This would be effective for sending out to the same few users, but if sending to external contacts/customers/vendors, this would be a lot of maintenance to keep up with.
If you have any questions or would like us to review the options with you, please reach out to us via a support ticket.