By the MailMail UK team
You paste a fresh temporary address into a sign-up form and the site answers with "Please enter a valid email" or "Disposable email addresses are not allowed". The address is fine: it can receive mail. The site has simply decided not to accept it. We run a temp mail service, so here is what happens behind that message and what you can do next.
How websites spot a temporary address
Domain blocklists
The most common check is a list of domains known to belong to disposable email services. Open-source lists on GitHub hold tens of thousands of domains, and many sign-up forms and email validation services use them or keep their own. If the part after the @ is on the list, the form refuses it before any email is sent.
Heuristics
Some checkers go further. They look at how a domain handles its mail and group domains that share the same mail servers, so a domain can be flagged even if nobody has added it to a list.
Patterns
A few sites also look at behaviour, such as many sign-ups from the same domain in a short time. These rules are private and change often, which is why an address can work on one site and fail on another.
Why sites block temp mail
- Free trials and promotions. A new address for every trial makes offers easy to abuse.
- Fake accounts and spam. Throwaway sign-ups cost nothing, so sites use the email step as a filter.
- Account recovery. A temporary inbox is public. If an account is tied to one, anyone who opens that address later can ask for a password reset. For accounts with real value, the block protects you as well.
What you can do
- Try another domain. MailMail offers 20 domains, and blocklists don't all cover the same ones. We add new domains regularly. Press Random, or type a username and pick a different domain. You can create up to five addresses a day.
- Use temp mail only where losing access is fine. Newsletters, downloads and one-off codes are good uses. An account you want to keep is not.
- Use an alias when a site insists on a real inbox. If your provider supports plus addressing, as Gmail does, an address like yourname+shop@gmail.com arrives in your normal inbox and is easy to filter, although some forms reject the plus sign too. Alias services such as Firefox Relay, DuckDuckGo Email Protection or Apple's Hide My Email give you a separate address that forwards to your inbox and can be switched off later.
Quick answers
Is my temporary address broken if a site rejects it?
No. The rejection happens on the website, before any email is sent. Your inbox still receives mail from other senders.
Why does the same domain work on one site and fail on another?
Each site uses its own list or validation service, and those lists are updated at different times.
Will a different domain always work?
No. Some sites block every address they can't confirm as long-lived. If several domains fail on the same site, it expects an inbox you will keep.
For more on when a temporary inbox is the right choice, see the questions on our home page.