email-verifier

email-verifier

✉️ A Go library for email verification without sending any emails.

Features

• Email Address Validation: validates if a string contains a valid email.
• Email Verification Lookup via SMTP: performs an email verification on the passed email (catchAll detection enabled by default)
• MX Validation: checks the DNS MX records for the given domain name
• Misc Validation: including Free email provider check, Role account validation, Disposable emails address (DEA) validation
• Email Reachability: checks how confident in sending an email to the address

Install

Use go get to install this package.

go get -u github.com/AfterShip/email-verifier

Supported Go versions

The three most recent Go releases, currently 1.25, 1.26 and 1.27, each covered by CI. go.mod requires the oldest of them, so an older toolchain will refuse to build this module.

Usage

Basic usage

Use Verify method to verify an email address with different dimensions

package main

import (
	"fmt"
	
	emailverifier "github.com/AfterShip/email-verifier"
)

var (
	verifier = emailverifier.NewVerifier()
)


func main() {
	email := "[email protected]"

	ret, err := verifier.Verify(email)
	if err != nil {
		fmt.Println("verify email address failed, error is: ", err)
		return
	}
	if !ret.Syntax.Valid {
		fmt.Println("email address syntax is invalid")
		return
	}

	fmt.Println("email validation result", ret)
	/*
		result is:
		{
			"email":"[email protected]",
			"disposable":false,
			"reachable":"unknown",
			"role_account":false,
			"free":false,
			"syntax":{
			"username":"example",
				"domain":"exampledomain.org",
				"valid":true
			},
			"has_mx_records":true,
			"smtp":null,
			"gravatar":null
		}
	*/
}

Email verification Lookup

Use CheckSMTP to performs an email verification lookup via SMTP.

var (
    verifier = emailverifier.
        NewVerifier().
        EnableSMTPCheck()
)

func main() {

    domain := "domain.org"
    username := "username"
    ret, err := verifier.CheckSMTP(domain, username)
    if err != nil {
        fmt.Println("check smtp failed: ", err)
        return
    }

    fmt.Println("smtp validation result: ", ret)

}

If you want to disable catchAll checking, use the DisableCatchAllCheck() switch (in effect only when SMTP verification is enabled).

 verifier = emailverifier.
        NewVerifier().
        EnableSMTPCheck().
        DisableCatchAllCheck()

Note: because most of the ISPs block outgoing SMTP requests through port 25 to prevent email spamming, the module will not perform SMTP checking by default. You can initialize the verifier with EnableSMTPCheck() to enable such capability if port 25 is usable, or use a socks proxy to connect over SMTP

Note: set FromEmail() and HelloName() before verifying at any volume. The defaults are [email protected] and localhost, and example.org is reserved by RFC 2606, so a server that validates the sender can reject the whole exchange before it ever considers the address you asked about. Rejections of this kind arrive at MAIL FROM and name the sender or the connecting host rather than the recipient, for example 550 5.7.25 Forward-confirmed reverse DNS failed or 550 5.7.1 Service unavailable, Client host [...] blocked using Spamhaus. Use a domain you control, with a PTR record for the IP you connect from.

Use a SOCKS5 proxy to verify email

Support setting a SOCKS5 proxy to verify the email, proxyURI should be in the format: socks5://user:[email protected]:1080

The scheme must be socks5 or socks5h, the only two golang.org/x/net/proxy resolves. Anything else, socks4 and socks4a included, fails the dial with proxy: unknown scheme; add it with proxy.RegisterDialerType before calling Proxy().

var (
    verifier = emailverifier.
        NewVerifier().
        EnableSMTPCheck().
    	Proxy("socks5://user:[email protected]:1080")
)

func main() {

    domain := "domain.org"
    username := "username"
    ret, err := verifier.CheckSMTP(domain, username)
    if err != nil {
        fmt.Println("check smtp failed: ", err)
        return
    }

    fmt.Println("smtp validation result: ", ret)

}

Two things to know about the proxy:

A query string in the proxy URI is ignored. golang.org/x/net/proxy reads only the scheme, credentials and host, so a ?timeout=5s has no effect. Use ConnectTimeout() and OperationTimeout() instead.

DNS does not go through the proxy. Only the connection to the mail server does, so MX lookups still leave from the local machine. To route them through the proxy as well, dial your DNS server over TCP through it — SOCKS5 carries TCP, and Go frames DNS over a non-packet connection per RFC 7766:

socksDialer, err := proxy.SOCKS5("tcp", "127.0.0.1:1080", nil, proxy.Direct)
if err != nil {
    return err
}
verifier = emailverifier.
    NewVerifier().
    EnableSMTPCheck().
    Proxy("socks5://127.0.0.1:1080").
    Resolver(&net.Resolver{
        PreferGo: true,
        Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
            // ignore network, which may be "udp": SOCKS5 CONNECT carries TCP only
            return socksDialer.Dial("tcp", "8.8.8.8:53")
        },
    })

Each lookup opens a TCP connection through the proxy, so this costs more than plain UDP DNS.

Use a custom DNS resolver

By default, the verifier uses net.DefaultResolver (the platform’s system-configured DNS resolver) for MX lookups and direct SMTP host lookups. You can override this behaviour by supplying your own *net.Resolver via Resolver(), for example to query a specific DNS server. When a proxy is configured, SMTP hostname resolution follows the proxy path rather than this resolver. MX record lookups always use this resolver, whether or not a proxy is set.

var (
    verifier = emailverifier.
        NewVerifier().
        EnableSMTPCheck().
        Resolver(&net.Resolver{
            PreferGo: true,
            Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
                d := net.Dialer{}
                return d.DialContext(ctx, network, "8.8.8.8:53")
            },
        })
)

func main() {

    domain := "domain.org"
    username := "username"
    ret, err := verifier.CheckSMTP(domain, username)
    if err != nil {
        fmt.Println("check smtp failed: ", err)
        return
    }

    fmt.Println("smtp validation result: ", ret)

}

Note: PreferGo: true is doing real work here, it is not boilerplate. MX record lookups always go through Go’s own DNS client, so a custom Dial is honoured for those either way. Resolving the mail server’s hostname to an IP for the TCP connection is different: without PreferGo that step may use the system (cgo) resolver, which ignores Dial entirely and quietly falls back to /etc/resolv.conf. Even with it set, /etc/hosts is still consulted before DNS, so a host listed there never reaches your resolver.

Misc Validation

To check if an email domain is disposable via IsDisposable

var (
    verifier = emailverifier.
        NewVerifier().
        EnableAutoUpdateDisposable()
)

func main() {
    domain := "domain.org"
    if verifier.IsDisposable(domain) {
        fmt.Printf("%s is a disposable domain\n", domain)
        return
    }
    fmt.Printf("%s is not a disposable domain\n", domain)
}

Note: It is possible to automatically update the disposable domains daily by initializing verifier with EnableAutoUpdateDisposable()

Suggestions for domain typo

Will check for typos in an email domain in addition to evaluating its validity. If we detect a possible typo, you will find a non-empty “suggestion” field in the validation result containing what we believe to be the correct domain. Also, you can use the SuggestDomain() method alone to check the domain for possible misspellings

func main() {
    domain := "gmai.com"
    suggestion := verifier.SuggestDomain(domain) 
    // suggestion should be `gmail.com`
    if suggestion != "" {
        fmt.Printf("domain %s is misspelled, right domain is %s. \n", domain, suggestion)
        return 
    }
    fmt.Printf("domain %s has no possible misspellings. \n", domain)
}

Note: When using the Verify() method, domain typo checking is not enabled by default, you can enable it in a verifier with EnableDomainSuggest()

For more detailed documentation, please check on godoc.org 👉 email-verifier

API

We provide a simple self-hosted API server script for reference.

The API interface is very simple. All you need to do is to send a GET request with the following URL.

The email parameter would be the target email you want to verify.

https://{your_host}/v1/{email}/verification

Similar Libraries Comparison

FAQ

The library hangs/takes a long time after 30 seconds when performing email verification lookup via SMTP

Most ISPs block outgoing SMTP requests through port 25 to prevent email spamming. email-verifier needs to have this port open to make a connection to the email’s SMTP server. With the port being blocked, it is not possible to perform such checking, and it will instead hang until timeout error. Unfortunately, there is no easy workaround for this issue.

For more information, you may also visit this StackOverflow thread.

The output shows "connection refused" in the smtp.error field.

This error can also be due to SMTP ports being blocked by the ISP, see the above answer.

What does reachable: “unknown” means

This means that the server does not allow real-time verification of an email right now, or the email provider is a catch-all email server.

Credits

• trumail
• check-if-email-exists
• mailcheck
• disposable domains from disposable/disposable-email-domains, both for the generated list and for EnableAutoUpdateDisposable(), less the few real providers it misclassifies (see cmd/build_metadata/disposable_allowlist.txt)
• free provider data from HubSpot’s free email domain list and tbrianjones/free_email_provider_domains, plus carrier and portal mailboxes vendored from Kikobeats/free-email-domains

Contributing

For details on contributing to this repository, see the contributing guide.

License

This package is licensed under MIT license. See LICENSE for details.