I recently installed mygpo on my YunoHost instance and, like several other users (e.g. Mygpo doesn't send verification email), stumbled upon the “missing verification email problem”. After checking (and re-checking) the email config and dabbling in the config files of mygpo to no avail, I finally found a solution:
Every ynh user is a mygpo user. You can just login with your regular ynh credentials.
While I haven’t found a fix for the verification email not being sent, and definitely feeling a lot dumber for not trying this earlier, I honored the KISS principle and simply created a new user with my desired email forwarding for mygpo.
Maybe this “workaround”/revelation can help somebody in the future.