Some email hosting sites are blocking emails sent by the Sync / Firefox Accounts server. Best I can offer is the suggestion to use a different email address from another domain, one that may be blocking. ''And if you tell us which domain you used, someone with Firefox Accounts services might be able to address your problem or have a more exact answer for you. The people providing support here are all volunteers, users like you; and we don't have access to the information to effect a fix.''