We are using odoo v12, but we experience the same problems. We've setup Google SMPT Relay service but only some emails are sent successfully. Others fail with the Error  :

Connection unexpectedly closed

We can't tell which emails fail, there is no logic behind this. Different recipients, different models, different timestamps.

This is very frustration, as we can't get any useful debug information from neither odoo.sh platform nor from Google SMTP Relay service.

Would be really grateful for any info helping us to solve this issue.

Hey there, I'm glad to hear you're also having this issue because I thought I was the only one, and going mad. I have been working with the Odoo technical team and showed them the issue - they are currently looking into this and promised a solution for us. I'll let you know when they get back to me.