I was able to email a participant using an administrator account. However, when a new user registers, the new user will not receive the mail. I have tried restarting service and commented out SMTP_USE_TLS = True. Both celery.log and indico.log show that the email was sent, but it was not received in fact.
We found out why. It’s because it looks like a scam was automatically filtered。You might want to consider changing your email subject line
Easier said than done, and it’s not like there’s just one spam filter out there so even if we changed it to make your filter happy, it might be caught by some other filter. 
I couldn’t agree more. I got it