Are Email Addresses Case Sensitive?

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Every email address has three parts. What comes before the '@' (the so-called "local mailbox part"), the '@' itself and what follows the '@' (the domain name).

The answer to the question whether email addresses are case sensitive—whether it matters if you type ReCipiENt@eXaMPle.cOm or RECIPIENT@EXAMPLE.COM or recipient@example.com—has to do with these elements of an email address.

The domain name part of an email address is case insensitive (i.e. case does not matter).

The local mailbox part, however, is case sensitive. The email address ReCipiENt@eXaMPle.cOm is indeed different from recipient@example.com (but it the same as ReCipiENt@example.com).

Simply put: Only the email mailbox name itself is case sensitive. Email addresses are not affected by case.

Case Typically Does Not Matter

Since the case sensitivity of email addresses can create a lot of confusion, interoperability problems, and widespread headaches, it would be foolish to require email addresses to be typed with the correct case. Hardly any email service or ISP does enforce case sensitive email addresses, returning messages whose recipient's email address was not typed correctly (in all upper case, for example).

This means that:

  • It does not typically matter what case you type an email address in when you send a message
  • If the recipient did give you an email address with distinct case, preserve it, however.

Help Prevent Email Address Case Confusion

If you send an email with the recipient's address spelled in the wrong case, it might return to you with a delivery failure. In that case, try to find how the recipient wrote their address and try a different spelling.

To minimize the risk of delivery failures due to case differences in your email mailbox name and to make the job easy for email system administrators, use only lower case characters when you create a new email address.

If you create a new Gmail address, for example, make it something like "j.smithe@gmail.com" and not "J.Smithe@gmail.com".

What the Standard Says

RFC 5321, the standard that defines how email transport works, lays down the email address case sensitivity issue thus:

The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. In particular, for some hosts, the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged. Mailbox domains follow normal DNS rules and are hence not case sensitive.

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