Like many websites, we use email addresses as user identifiers for logins.

RFC 5321 [#2.3.11] states that the local parts of email addresses:

MUST be interpreted and assigned semantics only by the host specified in the domain part of the address

This means that applications processing email addresses can't second guess, for example, the way that GMail will ignore dots when determining usernames. It also means that applications must treat email address local-parts as case sensitive, since that's how the local part is specified.

However users are stupid, and in a recent case a user who had signed up with an uppercase email address (and had successfully received and clicked the verification email) found they were unable to log in because they were now using the lowercase format of their email address.

I've never come across an email server which enforced case-sensitivity on its inboxes. I don't doubt that some exist somewhere, but I'm questioning whether there isn't a bigger benefit to dropping case-sensitivity for our email usernames than there are problems keeping it.

I wouldn't go so far as to emulate GMail's dot processing, or any kind of plus-addressing, because those don't seem to be as ubiquitous or as automatically-assumed as case-sensitivity. However, I'm aware it's not a black and white scenario so I'm interested if anyone knows how widespread case-sensitive email addresses actually are.

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    Two answers already from the same RFC. Next time, make sure you read the whole thing :)
    – yannis
    Jan 4, 2012 at 12:53
  • There are two answers to this question. Don't use email address as the username. Use the same string manipulation on both the user's input and the value stored in the database, if they don't match at that point, then they are not equal.
    – Ramhound
    Jan 4, 2012 at 14:40
  • Yahoo used to have case sensitive user names in the really, really old days. Like mid 90s. Made hacking yahoo games lots of fun . . . Jan 4, 2012 at 15:15
  • Why not just enforce case insensitivity on your user identifiers by converting them to an standard internal case during login? Your database can still store the case sensitive e-mail address for when you need to e-mail the user (for password recovery etc.). It would seem to be exceedingly unlikely that you would get two different users whose e-mail addresses only differ by the case of their local part.
    – Mark Booth
    Jan 4, 2012 at 16:16
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    @YannisRizos I did read it, but a SHOULD is always trumped in RFCs by a MUST. The fact that hosts SHOULD do something means that they might not and I just wanted reassurance that, in reality, the SHOULD is a bit more widely adopted than its definition implies.
    – Gareth
    Jan 4, 2012 at 17:43

3 Answers 3


https://www.rfc-editor.org/rfc/rfc5321#page-42 (emphasis added):

While the above definition for Local-part is relatively permissive, for maximum interoperability, a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form or where the Local-part is case-sensitive.

The rfc discourages case-sensitivity. Also, I personally have never seen a host with case-sensitive local parts.

Case-insensitive email addresses are a de-facto standard.

Therefore, I think you are correct to say there is bigger benefit to dropping case-sensitivity for your email usernames than there are problems keeping it.


What you should be doing is using the email address as provided for sending emails and transforming it into something less error-prone when using it as an account identifier or at least using it as a fallback when you don't find an exact match.

This would be a very rough analogue to what the Soundex algorithm does for words or names in English by removing the things that create ambiguities. For example, you might convert the entire address to lowercase and remove subaddresses and non-alphanumeric symbols from the local part (e.g., [email protected] would strip down to [email protected]). Applying the same transformation to any address used during login would get you a match for more variants, and the user is none the wiser because any other use of the address would be the as-provided version.

From a technical standpoint, this does make your logins marginally easier to brute force. If that's a concern, you can always require an exact match for logins and provide a "forgot my login or password" option that's tolerant when looking up the account since any email you send will be to the "right" address.

If an address like the one above makes it all the way through sign-up and verification, I don't think it would be unreasonable to reject others that transform the same way as duplicates. I'd posit that any site that assigns local addresses like Lance.Boyle, LanceBoyle, lance.boyle and lanceboyle and treats them as different might be the source of other kinds of trouble.

  • I'm not sure I'd go so far as to strip the dots - I think I'd stick to being case insensitive when checking logins - but fundamentally this is what I would suggest too.
    – Murph
    Jan 4, 2012 at 16:32
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    I definitely wouldn't strip the dots. Googling finds people named, for instance "Robert Oot" and "Richard Oot", they could well have email addresses like "[email protected]". Strip the dots out of that and you could well have a conflict. :-) Jan 5, 2012 at 4:45
  • That particular conflict isn't likely since the superuser is probably not out signing up for accounts on web sites, but you have a point. One possibility would be to only strip dots from domains where it's known not to matter and might be the source of a lot of users, like gmail.com.
    – Blrfl
    Jan 5, 2012 at 15:04

Paragraph 2.4. General Syntax Principles and Transaction Model of RFC 5321 states (emphasis mine):

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.

AFAIK all popular mail hosts avoid case sensitive emails, and you should too. I haven't used a service which assumed case sensitivity for emails, and I hate to be surprised.

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