Thank you @abrakadabra for your feedback and your blog post about how you solved this issue.
In fact, this specific issue with gmail is the point 5 of the email troubleshooting help page: Passbolt Help | Why are my emails not being sent?
We are aware the help site is very poor in terms of user experience and the fact you didn’t find the correct info confirm this.
I know a new help site is on the way, I hope it will be better than the current one in term of user XP.
Cheers,