There are a couple of email providers (I've found it a lot with Yahoo and ISP-exclusive email addresses) where the email actually gets delivered but the email provider returns a message that says the address was not found. Unfortunately, if the Firefox Account system gets one of those, it has no idea that it's untrue.

I guess the best option would be to open an account under a different email address from a different email provider (if possible) or contact your email provider for more help. Some providers have very strict "spam" filters.

Hope this helps.

Thanks for your reply, Wesley. You got me thinking about how the communication between Mozilla and our mailserver actually looks like. And since it's a company mailserver we're talking about, I have the luxury of looking into SMTP logs. Found out the user that was trying to sing up for Sync account had an automatic responder enabled, so I've temporarily turned it off, and it worked!