Validating an email address means checking that it’s real, active, and able to receive messages.
In this guide, you’ll learn how to verify an email address step by step. We’ll cover the basics of email validation, verification steps, tools and methods for email validation.
If you’re in a hurry, the easiest solution is to use an email validator – like this free online tool. It can instantly check if an email is valid and safe to use.
What is email validation?
Email validation is the process of confirming an email address is properly formatted and actually exists on a mail server, without sending an email.
It means checking that the address has a valid format (like name@example.com), belongs to a real domain, and ideally that the mailbox is ready to receive mail.
Why validate emails?
Because sending to invalid addresses hurts your email deliverability. Invalid emails will bounce back undelivered – a few bounces are normal, but a high bounce rate gives email providers negative signals about your sending practices.
By validating addresses, you ensure you only send to deliverable emails, protecting your sender reputation and improving inbox placement.
How to validate an email address
Validating an email address can be done automatically via email validation tools or manually by following a few sequential checks.
Email validation step-by-step
Below is a manual step-by-step process to verify if an email is valid.
1. Check the email’s format (syntax)
First, ensure the email address is formatted correctly. It should have a local username part, an @ symbol, and a valid domain part (e.g. username@example.com).
Look for common formatting errors: missing @ or dot in the domain, extra spaces, or illegal characters. For instance, an address with a space (john doe@example.com) or a missing TLD (john@example) is invalid.
Make sure the local part doesn’t start or end with a period and only uses allowed characters (letters, numbers, hyphens, underscores, and dots in the middle).
A quick way to do this is using regex or built-in validation functions.
2. Verify the domain name and DNS records
Next, check that the domain (the part after @) is a real, active domain.
You can use a DNS lookup to see if the domain exists and has MX records (Mail Exchange records) – these tell you if the domain has a mail server set up to receive email. If a domain has no MX record (and no fallback mail server via an A record), any email to it will go nowhere and hard-bounce.
Also, be cautious of domains that do exist but look suspicious or are known for spam/disposable emails.
3. (Optional) Perform an SMTP check for the mailbox.
For an even deeper verification, you can perform an SMTP handshake with the email’s mail server. A successful SMTP check can tell you the mailbox exists on the server without sending an email
Using an email validator tool
One of the easiest ways to check if an email is valid is to use an email validator tool. These online services combine all the essential checks into one process.
For example, Sidemail’s email validator (a free online tool) will check an address’s syntax, verify the domain and MX records, flag disposable email domains, catch common typos, and even do an SMTP server handshake.
It provides a clear result for each aspect (whether the address is formatted correctly, the domain can receive mail, if the address passed the SMTP check, etc.), plus an overall valid or invalid verdict.
Why use a dedicated validator?
Because it’s quick and reliable. Instead of manually performing multiple lookups and risking mistakes, the tool automates it.
Another advantage is that professional email validation services can handle bulk lists efficiently. If you have an email list of thousands, you can upload it and let the service bulk-validate all addresses at once.
Also, there are email list cleaner tools, that can help you remove invalid addresses from your list, further speeding up your process and improving deliverability.
Email validation methods explained
In this section, I’ll explain each key component of email validation in detail.
1. Syntax validation
Syntax validation is the first and most basic step – it ensures the email address is formatted correctly. The rules for email format are defined by standards (like RFC 5322).
To explain simply, you need a local part, an @ symbol, and a domain part.
The local part can include letters, numbers, and certain symbols (like periods, underscores, hyphens, plus signs), but it can’t start or end with a dot, and it can’t have spaces
The domain part has to be a valid domain name (letters, numbers, hyphens, dots separating labels) and typically includes a top-level domain like .com, .org, etc.
Syntax checking is a necessary first filter to catch typos and malformed addresses before doing deeper checks. Modern validators handle this for you. Just don’t rely on syntax validation alone – it’s just step 1.
2. Domain & MX record validation
After confirming the email looks properly formatted, the next question is – does the domain exist and can it receive email?
This step involves checking DNS records for the domain part of the email address.
Firstly, a DNS lookup for the domain will tell you if the domain is registered and active. If the domain doesn’t resolve (no DNS records at all), then the email address is not deliverable.
Secondly, and importantly, check for MX records on that domain. An MX record (Mail Exchange record) is a DNS record that specifies which mail server(s) handle email for the domain. If a domain has no MX record, it might still receive mail if it has an A record (as a fallback, some mail servers will try the domain’s A record), but lacking an MX is a strong sign the domain isn’t set up for email. In practice, legitimate email domains almost always have MX records. For example, example.com might have an MX record pointing to mail.example.com. If there are no MX records (and especially if there’s no A record either), any email sent there will bounce immediately.
A domain check should also flag domains that exist but might be problematic, for example disposable email domains. We’ll discuss these more below, but domains like mailinator.com or 10minutemail.net exist and have MX records, yet they’re used for throwaway addresses. A domain check alone won’t rule them out (since technically they can receive email), but a good validator will cross-reference against known disposable domains.
3. SMTP mailbox verification
This is the most direct check: asking the mail server if the address exists.
SMTP (Simple Mail Transfer Protocol) is the protocol through which emails are sent between servers.
Mailbox verification via SMTP goes further than DNS, it is the closest you can get to actually sending an email without sending one. It’s extremely useful, but requires more technical resources, and should be done carefully and with some caution as not all mail servers will give you a straight answer.
That being said, professional email validation tools are a great way to perform this type of verification without the technical overhead.
Double opt-in confirmation
It's important to note, that no method is 100% foolproof.
Email validation greatly reduces bounces, but it cannot guarantee delivery in every case. For example, some mail servers might initially say an address is OK but bounce the message later. Factors like full inboxes, temporary server issues, or spam filters can still cause a valid address to bounce.
That’s why double opt-in is the gold standard when feasible.
Double opt-in is when a user who signs up with their email is sent a confirmation email and must click a link or button to verify that their address is real and that they indeed want to receive emails from you.
Why is this important for validation? Because it’s the only method that confirms deliverability and user intent in one go.
From a deliverability standpoint, addresses obtained via double opt-in have virtually zero chance of bouncing, and you have proof of consent.
Summary
Validating email addresses is an essential practice for anyone who sends emails in bulk. It helps ensure that your messages reach real, active inboxes. By doing so, you protect your sender reputation, improve delivery rates, and save resources.
If you’re ever in doubt or need a quick answer on an email’s validity, don’t hesitate to use Sidemail’s email validator. It’s free, requires no signup, and combines all best-practice checks into one step for you.
Originally published at https://sidemail.io/articles/how-to-validate-email-address/
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