How to check for valid vs. invalid email addresses

Amy Elliott Amy Elliott
· 14 min read · October 11th, 2022
A list full of valid email addresses is crucial to make your campaigns successful, while invalid emails are dangerous for your deliverability. You need to deal with them before it’s too late!

Sending emails to invalid email addresses will result in your message hard bouncing. Continue sending to them and Internet Service Providers (ISPs) will take notice—your sender reputation and therefore your email deliverability will suffer. Before you know it, your emails will be headed straight to the spam folders of your valid subscribers too, or even worse: they’ll be blocked completely. 

Just as growing your email marketing subscriber list is an ongoing project, maintaining your email lists is too. In this article, we’ll take a look at invalid emails, why they’re bad for business, and how they can end up on your email list. Plus, we’ll share how to check for and prevent your email list from being infiltrated by harmful emails. 

When you reach the end you’ll be ready to tackle your list to keep it clean and healthy! Let’s go!

What is an invalid email address?


Invalid email addresses are email addresses that cannot or should not receive emails. For example, invalid emails that cannot receive emails include:

  • Incorrect formatting, like typos and syntax errors

  • The email address doesn’t exist

  • The mailbox is full and therefore cannot receive incoming mail

  • The mailbox is blocked by its service provider

There are also some invalid or risky types of email addresses that can receive incoming emails but you shouldn’t send to or should send with caution. 

  • Catch-all emails accept mail but may not be a real email address belonging to a person

  • Role-based emails are valid but may belong to a group or department

  • Disposable email addresses are temporary emails that people use to avoid receiving spam or marketing emails to their personal inbox—you shouldn’t send to these

Note: Invalid emails are different from inactive emails. Inactive emails are valid email addresses belonging to subscribers who haven’t engaged with your emails for a significant amount of time. It’s a good idea to segment these subscribers, try to win them back and exclude them from future email campaigns

How do invalid email addresses end up on your list?

There are a few ways that invalid emails can make their way to your email list, some of which are completely avoidable. Others are sometimes out of your control, but that’s where steps to check and prevent invalid emails come in—we’ll talk about this later.

1. Using a purchased email list 

This is the number one way to contaminate your email list with bad email addresses. Subscribers that are unlucky enough to end up on a purchased list have not consented to be subscribed to your emails. What’s more, these lists are full of spam traps, fake emails and inactive mailboxes. 

As tempting as it may be, purchasing an email list should be avoided at all costs. If you need more convincing, here’s 7 reasons you should never buy an email list.

2. Disposable emails

Many people use disposable emails to sign up to services or receive freebies so they can keep their inbox free of marketing emails and spam. These are risky because while they might be delivered at first, they have no engagement and often become inactive after a period of time.

3. Human error

We all know the saying that to err is human. The same can be said when it comes to your email list. Typos and errors when entering an email are out of your control, so some are bound to slip through the cracks. Luckily, there is a way to prevent such email addresses from making it to your list, and we’ll talk about this later.

4. Outdated subscriber details

It’s normal that with time, your email list will decrease in quality. People change their contact details all the time, whether they’ve lost access to their account, left the company to which the email address belonged to, or simply outgrew their teenage email address. Oftentimes, subscribers forget to update their details meaning you’re left with the inactive email address. 

5. Full mailbox

If you’re anything like me, you probably don’t keep your personal inbox very organized (I may have shocked a few people who caught a glance at the number of unread messages). As the emails pile up and the mailbox reaches capacity, it’s likely to reach a point where it will stop accepting new messages. This causes incoming emails to bounce. 

6. Dreaded spam traps

Even if you’ve built your email list by the book, you’re not completely immune to spam traps. ISPs often use inactive email addresses as spam traps to catch out spammers and senders who are lazy about keeping their email list clean. Additionally, there’s always the chance that someone signs up to your list with a spam trap on purpose to cause harm to your email list. 

Examples of invalid email addresses

If you’re unsure about whether an email address is invalid, there are a number of things you can do to test it out. But if you’re manually checking for errors or typos, here are a few examples of invalid email formats to show you what to look for. 

  • Obvious typos: [email protected]

  • No @ symbol:

  • The local part, or recipient name is missing:

  • The email domain name (example) is missing: [email protected]

  • The top-level domain (.com) is missing: paul@example

  • A special character at the beginning or end of the email address: [email protected]

Why invalid emails are dangerous

If you send emails to invalid addresses you’ll get a hard bounce. The higher your hard bounce rate (anything over 2% is considered problematic) the higher the chance that your sender reputation will be negatively affected. In turn, this will impact your email deliverability meaning more of your messages will head to the spam folder. 

You could even find yourself on an IP blocklist, some of which are extremely tricky to get removed from. (Check out our guide If you’re interested in learning more about email blocklists and how to get removed from them.)

This is because when you have a high bounce rate, spam filters and ISPs take this to mean that you’re a risky sender—either because you’re sending spam or not following best practices by keeping your email list clean or abiding by spam/privacy laws such as GDPR.

As well as being bad for your deliverability, invalid emails also:

  • Are a waste of money. If you’re sending to email addresses that will never receive or open your messages, it will still cost you. Keeping invalid email addresses on your email list means you’ll pay more to Email Service Providers (ESPs) for subscribers that don’t exist. 

  • Will negatively impact your ROI. The more you spend on emails that don’t convert, the lower your Return on Investment.

  • Skew your email campaign metrics. Without knowing how many emails in your list are invalid, your metrics won’t be reliable. The cleaner your list, the better the data, meaning you’ll have a clearer picture about your email marketing campaign performance and be able to make informed decisions about your strategy.

How to check if an email address is valid

If you’re collecting subscribers and sending emails, it’s important to regularly clean your email list to avoid harming your deliverability. There are a few different ways you can validate an email address, some are easier and more reliable than others—let’s take a look.

1. Send an email

If you have just one, or at most a few, email addresses you’re unsure about, you could always check if they’re valid by simply sending them an email. It’s not recommended that you do this for many email addresses that you suspect are invalid. 

The best way to do this is to just ask the subscriber to confirm their email address. If any emails are delivered, give them a few days to confirm. You can then remove any subscribers that don't confirm from your list.  

Otherwise, any emails sent to fake addresses will bounce back and you’ll receive an error message confirming that the email couldn’t be delivered. It will also include technical details about the reason for the bounce.


2. Password recovery

The password recovery method involves heading to the recovery pages of email providers like Gmail, Yahoo! or Microsoft Outlook and entering the email address to see if it exists. If the email address is invalid, you’ll receive an error message telling you that the account doesn’t exist, just like below.


Again, you should only use this method if you have a small number of emails to check to avoid your IP being blocked for suspicious activity. It would also be super time-consuming to check a ton of emails this way!

3. Do a quick Google search

You can quickly get an idea of whether an email address is valid by searching for it on a search engine like Google. Most people have an online presence on websites or social media such as LinkedIn and therefore their email address can be found online.

This method won’t necessarily tell you whether an email address is in use by the person that entered it, but it could give you an indication of whether it is deliverable. What’s more, if the email address has been associated with a spam trap or some sketchy behavior, your search might reveal this as well.

4. The best method: Use an email verification tool

We may be biased, but actually - we’re not! An email verification tool is the quickest, most convenient and reliable way to validate your email list. Not only can you verify a single email address or upload an entire list, but it will also give you the status of each email. This means you’ll be able to identify which email addresses are disposable, catch-all, or have typos and easily remove them from your list. 

How to verify email list with Mailercheck

MailerCheck is a powerful email validation service that can verify your whole list in minutes. Additional features like Email Insights and Inbox Placement help you to optimize your deliverability even further by checking your email content and testing to see if your emails will reach the inbox of leading mailbox providers like Gmail, AOL, Yahoo! and more. 

You can sign up to MailerCheck for free and get 200 credits to start out with.

How to avoid invalid email addresses appearing on your list

You can’t control whether someone will try to sign up to your list with a fake email address or enter a typo by mistake, but you can control whether or not their sign-up is successful. The answer lies with an email verification API.

An email verification API will enable you to connect your app or website to an email verification tool so that when a user enters their email address, it’s instantly verified. You can use this function to automatically send invalid emails to a suppressions list, or even better, provide an error message to the user telling them that the email address they have entered is invalid. It helps to keep your email list clean and provide real subscribers with a better user experience. 

MailerCheck has its very own email verification API that you can connect via our well-supported, custom API or Zapier integration.

Invalid emails are here to stay

As long as emailing exists, invalid emails will too. It’s on you, the sender, to regularly clean and properly maintain your email list and remove invalid emails—think of it as an essential part of your email strategy. Using an email verification tool is the quickest and easiest way to check your list—it will help you to protect your sender reputation and deliverability while saving money with your ESP.

What steps do you take to avoid or remove invalid email addresses from your email list? Let us know in the comments!

Verify your email list today!

Join MailerCheck, the email verification tool backed by deliverability experts, and get 200 free credits plus access to Email Insights and Inbox Placement testing.

Amy Elliott
I’m Amy, Content Writer at MailerCheck. As a child, I dreamt about writing a book and practiced by tearing pages from an A4 notepad and binding them with sugar paper. The book is pending but in the meantime, I love taking a deep dive into technical topics and sharing insights on email metrics and deliverability.