Are Email Addresses Case Sensitive?

When I spell out an email address to someone, often people ask “all in small letters?”… or “all together?”. Even in these times when millions of emails are sent everyday around the world, a number of people are still unaware of basic information about basic technology! Today, we will discuss whether email addresses are case sensitive.

For all practical purposes, case does not matter in email addresses. Conventionally all the email addresses are written in small letters but you can write/type them in capital letters as well.

If we get into technicalities, theoretically case does matters in email addresses (but practically it does not, as we have already said). For example, practically [email protected] is same as [email protected] or [email protected]. However, theoretically all these email addresses are not the same.

RFC 5321 is the standard that defines how email should work. Let us see what the standard RFC 5321 has to say about case-sensitivity of email address:

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.

As per the standard the “mailbox” must be treated as case sensitive. But what is a mailbox? Each email address has three parts. What comes before the ‘@’ is the “mailbox” including the ‘@’ itself. Mailbox usually refers to the user/owner of the email address. What follows the ‘@’ is the domain name. So as per the standard, [email protected] is not same as [email protected] but is same as [email protected]. In other words, only the email mailbox name itself is case sensitive but the domain name of the email address is case-insensitive.

Every email address has three main parts. It’s useful to understand the components of an email id.

Case Does Not Matter

Email addresses are not affected by case. No email service or ISP enforce case sensitive email addresses. For example, messages bounce for various technical reasons including wrong spelling or using an invalid special character, like using a ‘dash (-)’ instead of ‘under-score (_)’. But never did an email bounce because the email address was in all upper or lower case. I have tried various combinations of lower and upper case characters in an email address and exchanged mails between Yahoo, Gmail and Rediffmail. All test emails reached the recipients and not a single mail bounced back.

However, to follow convention and to avoid confusion you should always create a new email address using only lower case characters. For example: [email protected] is more acceptable than [email protected] or [email protected].

Conclusion

Email addresses are not case-sensitive. But, if the recipient email address is written with a distinct case, use it as it is. It is advisable to always use lower case characters while creating a new email address.

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