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.
UPDATE: AUGUST, 2018
Google Cloud Platform has opened up ports 465, and 587 for usage. We have tested success with the following configurations:
Office 365: If the user currently uses Office 365 (E3 or Business Essentials) their account will have access to use the Office 365 SMTP server. I have tested this with Sage 300, Sage 100, and QuickBooks.
authentication: Office 365 username and password. If they have 2FA turned on, you'll need to use an App Password.
GMail: Normal GMail or GSuite users will be good to use their respective SMTP server.
authentication: GMail username and password.
GoDaddy: GoDaddy does not allow port 587 to be used, and most of these business applications will not allow SSL properly. In the case I have configured GoDaddy, I configured it as shown below
authentication: GoDaddy webmail username and password