Email Addresses: The Question of Case Sensitivity

The inbox associated with an email address is a locked box – only unlocked when an email is sent with that specific email address in the recipient field. What many people wonder is simple – does the key to this locked box has to be exactly right? Or is there some room for error? In other words, does character case hold any weight when it comes to the validity of an email address? Every email address has two distinct sections – the username, followed by an @ for separation, and then the name of the domain the email address is registered at, along with the top-level domain. The question is, if the email address an email is intended for is, will sending the email to or (or any other variation of the email address with characters in upper case) send the email to the intended inbox or send it to a completely different email address (or simply return a Delivery Failed message in the event that the unintended recipient email address doesn’t exist)? Is either part of the average email address case sensitive?

The Universally Established Precedent

Email is a universally maintained and functioning network, not some haphazard, half-baked piece of virtual infrastructure. Every single part of the world’s email network has been carefully mapped out and precedents and standards for every single aspect of it have been established. RFC 5321 is the standard that deals with everything there is pertaining to email transport, and it has quite a bit to say about case sensitivity in email addresses:

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 – RFC5321

There you have it – according to, well, email law, email service providers are supposed to treat the local username part of an email address as case sensitive because not doing so will almost always lead to a significant amount of confusion and hinder operations, but domain names and extended top-level domains are exempt from case sensitivity. Pretty cut and dry, don’t you think? Not really, because that’s not the whole story. The law is only a part of the conversation – the other part is what’s actually going on around the world and just how the law is being implemented in email transport.

The Practically Applied Precedent

The universally established and recognized precedent dictates that the domain name be treated as case insensitive, whereas the local username registered on the domain in question be treated as case sensitive. That would mean that the email address is the same as recipient@dOmAiN.coM but not the same as This, however, is not always true. You see, the case sensitivity of email addresses actually varies from one email service provider to the next. Case sensitive email addresses, even if only the local username part of them is case sensitive, can lead to a lot of confusion, not to mention the risk of interoperability problems and an array of different headaches for service providers. That being the case, a lot of the email service providers out there choose to forego the email address case sensitivity precedent and either fix character case for their clients or ignore character case altogether, in which case both upper case and lower case characters are perceived to be the same by the network.

What this basically means is that most email service providers don’t have their clients fretting over what case they type the characters that make up the email addresses they want to communicate with. If you’re lucky enough to be using one of these email service providers, when you’re sending an email to a specific email address and any of the characters are supposed to be upper/lower case but you don’t type them as such, the email will still make its way to the right mailbox – it won’t end up in the wrong inbox or be returned to you for having an invalid email address.

Dealing with Case Sensitivity in Email Addresses

Unless the email service provider you or the intended recipient of the email is using is a real stickler for the rules and enforces case sensitivity in usernames, the case you type the recipient’s email address in won’t matter. However, if the recipient communicated their email address to you with any parts of it in upper (or lower) case, the recommended course of action is to preserve the character case that was communicated to you in order to avoid any confusion and to minimize the risk of a failed email delivery. If you’re creating a new email address, only use lower case characters – believe me when I tell you your email service administrator and every single person who ever has to send an email to you will thank you for it. Use special characters (such as  and _) to maintain the individuality of your email address, not upper case characters. Upper case characters in email addresses are simply an unnecessary and easily avoidable nuisance, and they don’t reflect well on their owners either.

An Interesting Tidbit

Most email service providers are doing the world a favor by being lenient with alphabet case in email addresses. However, Google, in Google fashion, is one-upping all of them by even disregarding periods in both the username part and the domain part of their email addresses. This means that, to Google’s email system, and are all the same email address!


Kevin Arrows

Kevin Arrows is a highly experienced and knowledgeable technology specialist with over a decade of industry experience. He holds a Microsoft Certified Technology Specialist (MCTS) certification and has a deep passion for staying up-to-date on the latest tech developments. Kevin has written extensively on a wide range of tech-related topics, showcasing his expertise and knowledge in areas such as software development, cybersecurity, and cloud computing. His contributions to the tech field have been widely recognized and respected by his peers, and he is highly regarded for his ability to explain complex technical concepts in a clear and concise manner.