When sending email from Manager, are you sending it from @yahoo.com email address to another email address under @yahoo.com domain? If so, Yahoo will quietly discard the email.
Try to send email from Manager without modifying From email address. Leave it as no-reply@manager.io and see if the email arrives.
Upcoming version of Manager will not allow to set custom From email address unless custom SMTP server is used. This is to avoid email delivery issues some people have been complaining about.