How to receive email on Google Compute Engine

TL;DR - You're right outgoing email ports are blocked, but incoming email ports are not. If you open them, and run a mail server which listens for incoming emails - you can still receive them. You will need to use 3rd party email sender gateways like SendGrid only to send the emails.

Longer version

All outgoing traffic to port 25/465/587 are blocked so that people do not abuse GCE for sending out spam, phishing, etc. emails.

You can still run a mail server which just receives emails. In other words, there is no mention that incoming traffic to those ports are blocked. By default GCE firewall blocks all incoming traffic. You will have to open up these listening ports using firewall rules.