contournement trouvé: cf https://github.com/YunoHost-Apps/opensondage_ynh/issues/95
By using mail without SMTP which just works on Yunohost, which will then mail as local 'opensondage' Linux user
# edit /var/www/opensondage/app/classes/Framadate/Services/MailService.php
# comment out the function that configures SMTP
//$this->configureMailer($mail);
& restart php
yunohost service restart php8.2-fpm