Are email addresses case sensitive?

Having invalid email addresses on your list can cause bounces and prevent email from reaching your leads. Incorrect syntax causes many validity failures, and often comes up as a source of confusion for email marketers. In this article we address a common grey area: uppercase letters and, in general, case sensitivity in email addresses.

Are email addresses case sensitive?

The short answer is technically yes. However, typically email addresses are not case sensitive: if you type your email address into a form and accidentally capitalize a letter or two, it probably won't jeopardize the sender's ability to email you. 

We must explore the issue in more depth to get a clear answer: to shed some light, let’s first look at basic email address structure, and then discover the different rules between the local part and the domain part.

What are the different parts of an email address?

 An email address is composed of two main parts:

  • local part - the part before the @ symbol
  • domain part - the part after the @ symbol

When it comes to case sensitivity, the rules differ between each part.

Is the domain part case sensitive?

No, in accordance with RFC 1035, the domain part of the email address is case insensitive.
That means you could type in:


and both would return the same results. Similarly, regardless of the email service provider, if a user capitalizes a letter by mistake in your entry form, it's still considered valid.

Is the local part case sensitive?

Yes, according to RFC 5321, the local part is case sensitive. However, Email Service Providers (ESPs) widely recognize that allowing upper case letters can lead to unnecessary confusion. That’s why most ESPs limit the options available to users when creating an email address.

Basically, while ESPs technically can give their users that flexibility, they typically choose to not to, in an effort to maintain consistency and standardization.
That means:

may return different results from

depending on the email service provider.
But in most cases, both would reach the same recipient.

How can I learn more about email address syntax?

At Verifalia we've covered some common syntax issues in depth in our knowledge base, so check out these articles.

Just remember: while it can't hurt to familiarize yourself with basic syntax standards, consider these articles a trusty reference. You'll save yourself plenty of time and headaches by verifying syntax with Verifalia, rather than trying to check each address manually.

Invalid syntax can cause emails to bounce, but as an email marketer you may face a variety of other validity issues that can affect your deliverability. Check out our knowledge base articles to learn about more complex issues, like greylisting and SMTP tarpitting.

How does Verifalia flag invalid email addresses?

At Verifalia, our advanced email validation technology can screen email addresses for a gamut of issues.
One Verifalia validation pass checks for: 

  • Accurate syntax, including support for Internationalized Domain Names (IDN) and mailboxes, meaning we can even validate non-ASCII email addresses
  • Role accounts
  • Domain / DNS records
  • Disposable email addresses (DEA)
  • Mail exchangers (MX) validity
  • Catch-all addresses
  • Non-existent mailboxes (including Yahoo! support)
  • Temporary unavailability
  • Honey Pot detection

Verifalia’s email validation service gets you clear, accurate results in a friendly, modern interface. Try us out now with our free demo.

Ready to start validating your list? Register with Verifalia now.

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