How to setup a mailserver on Google Cloud VM?
According to https://cloud.google.com/compute/docs/tutorials/sending-mail/, you cannot set up a mail server the usual way, as ports 25, 465 and 587 are blocked for outbound connections on Google Cloud. Instead, you might take a look at relaying services such as Mailgun or SendGrid, which allow sending through port 2525 or an API instead. These services might cost a little bit of money, however.