Do Capital Letters Matter in Email Addresses?

Every email address has two parts that are separated by the @ sign; the username followed by the domain name and top-level domain where the email account belongs. The question is whether or not case sensitivity matters.

For example, is recipient@example.com the same as ReCipiENt@example.com (or any other case variation)? What about recipient@EXAMPLE.com and recipient@exAMple.com?

The domain name part of an email address is case insensitive (i.e. the case does not matter). The local mailbox part (the username), 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 username itself is case sensitive. Email addresses are not affected by the case.

However, this isn't always true. 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. This is why some email providers and clients either fix the case for you or ignore the case altogether, treating both cases as equals.

Hardly any email service or ISP enforces case sensitive email addresses. This means even if the letters are supposed to be upper/lower case but are not, the emails are not returned as invalid.

Here's what this means:

• It typically does not matter which case you type an email address in when you send a message
• However, if the recipient did give you an email address with distinct case, preserve it
• You should always only use lower case characters when creating a new email address to rule out any 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. Replying to the message, for example, should let the email go through because you'll be replying to the exact same address that emailed you.

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 instead of J.Smithe@gmail.com.

Tip: Google email addresses are actually really interesting because they ignore not only letter case in the username and domain portion, but also periods. For instance, jsmithe@gmail.com is the same as j.smithe@gmail.com, jsmi.th.e@gmail.com, jSm.iTHE@gmail.com and even j.sm.ith.e@googlemail.com.

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.