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The part before the @ in email addresses is case sensitive

Posted by jpluimers on 2018/10/16

At [WayBackError when trying to signup using an email address with uppercase letters (#27898) · Issues · / GitLab Community Edition · GitLab, I commented this:

Both the :e-mail and :email_confirmation fields should get the same case processing treatment.

That treatment should consist of this:

  1. The part before the @ should be treated as case sensitive
  2. The part after the @ should be treated as case insensitive

This means that:

  • Foo@Example.Org and are the same
  • and are different

The main reason is that there are email systems expecting case sensitivity in the part before the @ sign.

I think excluding those users from being able to use GitLab is a bad idea.

See especially the comments at the Stack Overflow answer to Are email addresses case sensitive?

Relevant RFC 5321: Simple Mail Transfer Protocol sections:

Important comments:

I work at a large company and there is another person with the same first and last name. I discovered today that his local-part differs from mine only in capitalization. This has been working properly, so I was surprised to see “no widely used mail systems distinguish different addresses based on case”. We use MS Exchange which I would call “widely used”. – Matthew James Briggs Nov 24 ’15 at 20:14

RFC 5321 2.4. General Syntax Principles and Transaction Model – 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”. Mailbox domains follow normal DNS rules and are hence not case sensitive. – Adam111p Apr 27 ’16 at 10:02

Most important parts of the answer:

From RFC 5321, section-2.3.11:

The standard mailbox naming convention is defined to be “local-part@domaiN“; contemporary usage permits a much broader set of applications than simple “user names”. Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address.

So yes, the part before the “@” could be case-sensitive, since it is entirely under the control of the host system. In practice though, no widely used mail systems distinguish different addresses based on case.

The part after the @ sign however is the domain and according to RFC 1035, section 3.1,

“Name servers and resolvers must compare [domains] in a case-insensitive manner”


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