From the FAQ [1]:<p>> Why won’t a site accept my Relay alias? Some sites [...] have stopped accepting all addresses except those from Gmail, Hotmail or Yahoo accounts.<p>I think a more likely explanation is the fact that many sites these days block the usage of email addresses from domains known to host disposable addresses. For example, mozmail.com (the top domain that is used by Firefox Relay) appears on [2] that is sourced by many libraries on GitHub.<p>[1] <a href="https://relay.firefox.com/faq" rel="nofollow">https://relay.firefox.com/faq</a><p>[2] <a href="https://github.com/disposable/disposable-email-domains" rel="nofollow">https://github.com/disposable/disposable-email-domains</a>