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Library to validate email for format, common typos and one-time email providers


~> 2.0
~> 10.0
~> 3.9.0
~> 0.78.0
~> 1.37.0


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EmailInquire is a library to validate email for format, common typos and one-time email providers.



Before a user is a user, they are a visitor. And they must register to be so. What if they makes a typo while entering their email address during the registration ? If they didn't notice, you just lost them. They won't be able to sign in next time.

Your users :

  • may not be as tech savvy as you;
  • may not remember exactly their email address;
  • may make a typo while typing their email address (very very common on a mobile keyboard).

While we can't do so much for the name part of the email address, for the domain part, we can be smart!

And also, we don't want for users to use one-time email addresses (also called burner or disposable email addresses).

Supported cases

All supported cases are based on a static validation. The gem does not check (yet) for domain existence (DNS) and prior delivery (MX entries on DNS).

Email format

This doesn't strictly follow RFC 5322, it aims at validating email that will be deliverable on Internet. It also takes into account length of email, name part and domain part as per SMTP specification.

  • foo@domain..com => invalid
  • foo@my..domain.com => invalid
  • foo@my--domain.com => invalid
  • foo@localhost => invalid
  • foo@ => invalid
  • secrétariat@domain.com => invalid
  • foo+test@domain.com => valid
  • ...


One char typo for 43 common email providers (worldwide and from France, United Kingdom and USA):

  • gmil.com => hint gmail.com
  • hitmail.com => hint hotmail.com
  • outloo.com => hint outlook.com
  • virinmedia.com => hint virginmedia.com
  • ...

ccTLD typos

ccTLD specificity, like United Kingdom .xx.uk domains:

  • foo.couk => hint foo.co.uk
  • fooco.uk => hint foo.co.uk
  • yahoo.uk => hint yahoo.co.uk
  • foo.judiciary.uk => ok!
  • foo.uk => ok, .uk is open to registration
  • ...

...and same thing with .co.jp & .com.br domains.

Email provider

Providers with an unique domain:

  • gmail.fr => hint gmail.com
  • gmail.de => hint gmail.com
  • google.com => hint gmail.com
  • free.com => hint free.fr
  • laposte.com => hint laposte.net
  • laposte.fr => hint laposte.net
  • ...


4700 one-time email providers a.k.a. burners, or disposable email (source):

  • yopmail.com => invalid
  • ...

Known invalid domains

  • example.com => invalid

Custom invalid domains

Add your own invalid domains:

# in config/initializers/email_inquire.rb
EmailInquire.custom_invalid_domains << "bad-domain.com"
  • bad-domain.com => invalid

Custom valid domains

Take precedence over all the above rules and make any domain in the list valid, and non hintable. Add your own valid domains:

# in config/initializers/email_inquire.rb
EmailInquire.custom_valid_domains << "good-domain.com"
  • good-domain.com => valid


Add this line to your application's Gemfile:

gem 'email_inquire'

And then execute:

$ bundle

Or install it yourself as:

$ gem install email_inquire


Use EmailInquire.validate(email), you'll get a EmailInquire::Response that represents weither or not the email address is valid or may contain a mistake.

Methods of EmailInquire::Response:

Method Description Possible values
#email The validated email address "john.doe@gnail.com"
#status The status of the validation :valid :invalid or :hint
#valid? Is the email valid ? true or false
#invalid? Is the email invalid ? true or false
#hint? Is there a possible mistake and you have to show a hint to the user ? true or false
#replacement A proposal replacement email address for when status is :hint "john.doe@gmail.com" or nil


A valid case:

response = EmailInquire.validate("john.doe@gmail.com")
response.status # :valid
response.valid? # true

An invalid case:

response = EmailInquire.validate("john.doe@yopmail.com")
response.status   # :invalid
response.valid?   # false
response.invalid? # true

A hint case:

response = EmailInquire.validate("john.doe@gmail.co")
response.status      # :hint
response.valid?      # false
response.hint?       # true
response.replacement # "john.doe@gmail.com"

A custom invalid case:

# in config/initializers/email_inquire.rb
EmailInquire.custom_invalid_domains << "bad-domain.com"


response = EmailInquire.validate("john.doe@bad-domain.com")
response.status   # :invalid
response.valid?   # false
response.invalid? # true

A custom valid case:

# in config/initializers/email_inquire.rb
EmailInquire.custom_valid_domains << "example.com" # would be otherwise invalid
EmailInquire.custom_valid_domains << "sfr.com" # would be otherwise hinted to "sfr.fr"


response = EmailInquire.validate("john.doe@example.com")
response.status   # :valid
response.valid?   # true
response.invalid? # false

response = EmailInquire.validate("john.doe@sfr.com")
response.status   # :valid
response.valid?   # true
response.invalid? # false


I think it's important to just offer a hint to the user and to not automatically replace the maybe faulty email address in the form.

A "Did you mean xxx@yyy.zzz ?" has the following advantages:

  • user remains in charge: we could have hinted against a perfectly valid email;
  • user is educated;
  • mini whaoo effect;

This "Did you mean xxx@yyy.zzz ?" is better being actionable, and appearing to be so: a click or tap on it should replace the email by the suggestion.

  +---------------------------------------+  +---------+
  | john.doe@yaho.com                     |  | Sign Up |
  +---------------------------------------+  +---------+
    Did you mean john.doe@yahoo.com ?

Note that you could even have this validation for your Sign In form...


Why does a perfectly valid corporate domain is hinted ?

There are a few cases of corporate domains that will be hinted to the related public provider domain:

  • google.com => hint gmail.com
  • laposte.fr => hint laposte.net
  • sfr.com => hint sfr.fr

This is intended. Taking google.com (corp) vs. gmail.com (public provider):

  • there are far more people with a gmail.com address rather than people with google.com address;
  • employees of Google are well aware of the difference between google.com (their employee address) and gmail.com (the public email provider offered by their company) and will not be mistaken by a hint;
  • non-tech savvy people are not, and have in mind "my email address is google", so not hinting to gmail.com would let a lot of actual errors pass.

If you do not want this, add the affected domains to EmailInquire.custom_valid_domains.


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Mutation testing

We use mutant to ensure that everything is well tested and the code is minimal. Coverage reported should be close to 100%.

Run it with:

bundle exec mutant --include lib --require 'email_inquire.rb' --use rspec -- 'EmailInquire*'


Bug reports and pull requests are welcome on GitHub at https://github.com/maximeg/email_inquire.


The gem is available as open source under the terms of the MIT License.