Before Opensolr sends you a registration code, it checks the address you typed. Throwaway and freshly made domains are a strong sign of abuse, so some signups are refused.
01 · What is checked
Where you connect from
Registration is not open from every country.
Domain reputation
A reputation service flags domains known for disposable addresses, spam, blocklists, or with no mail server.
Disposable providers
Known temporary email providers are refused, from a list that is updated every day.
The DNS of the domain
A domain must be able to receive mail (MX records). SPF and DMARC records and the age of the domain also count: a very new domain with neither SPF nor DMARC is refused.
02 · What to use instead
An address on an established domain: your company domain, or a well-known provider such as Gmail, Outlook, Yahoo Mail, Fastmail, Zoho Mail or iCloud.
03 · Think it is a mistake?
Write to support@opensolr.com with the address you tried. We review it by hand.