Avoid Capital Letters in Email Addresses

Categories: Hosting

Using capital letters in email addresses is something I've noticed people doing for a few years now. But it's not a trend that you should consider following since I've seen some of the problems this creates first hand.

For the most part, people aren't concerned with the difference between john.doe@example.com and John.Doe@Example.com for example, but there are some systems and software where that difference is like night and day.

Mail Lists

Some mail lists will only allow subscribers to send messages if they are a member. Some of these lists, specifically the popular Mailman, are case sensitive. If john.doe@example.com (all lowercase) is a member of the mailing list and his email program is configured to send out as John.Doe@Example.com (with capital letters), his messages will be bounced as the mailing list doesn't recognize him as a member.

Spam Filters

While I don't have any proof of this, I do believe some spam filters and white lists may not work properly if they have rules setup for an email address that is all lowercase, and messages are arriving from an email address using upper case, or vice-versa.


For many years email addresses, and internet domain names, have always been lower case. People who have your contact in their address book likely has it all lower case. Now if you go and change things by capitalizing you email address, you inadvertently add another separate entry in their address book. Now multiply this action by a few more contacts doing the same and suddenly that address book is full of duplicate entries which some would find frustrating to deal with.

Company Listings

Some places where your addresses are listed may end up listing some as lower case, others as capitalized, and when displayed together will look inconsistent and amateur. This is a smaller concern, but it has been known to happen, especially in systems that let users register themselves; some users may use their capitalized versions, others may not.


Gravatar is a service that many blogs and other systems use in their commenting systems. It allows you to setup a profile picture to be used next to your comments based on your email address. Gravatar itself is case sensitive, so if someone sets up a Gravatar account for John.Doe@Example.com, but then posts comments as john.doe@example.com, their profile picture will not be displayed. Which may result in John using John.Doe@Example.com for the most part online and creating inconsistency with the john.doe@example.com printed on his business cards.

While Gravatar is just one service, this may be the case for other services as well. It may even effect what email address you use to login to social sites such as LinkedIn, Facebook, and Twitter.

Super Geeky Technical - RFC 2821

To get super geeky and technical, RFC 2821, which defines how email SMTP servers should behave, specifically notes that the local part of an email address, the part before the @example.com, must be considered case sensitive. While it seems a majority of SMTP servers do not follow this, they should, since it is part of the RFC which defines how they should behave.

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. Mailbox domains are not case sensitive. In particular, some hosts treat the user "smith" differently from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged. http://www.ietf.org/rfc/rfc2821.txt


Most of the time there won't be any issues at all using capital letters or not. However there are some cases that may have a negative effect. Overall, you need to ask yourself why you're going with capital letters and if it will really bring any clarity to someone reading that email address. Most times the case is not strong enough for capitalization, it's just done without thinking about the potential issues it may cause. Many times, when clarity is the reason for capitalization, a better option is to change the email address to something that uses periods or dashes to break up first/last names, or first/last initials. johndoe@example.com becomes john.doe@example.com or john-doe@example.com.

Your Thoughts?

I'd love to know what your thoughts are on the subject. Is there anything I missed? Any more examples of other systems that are affected by email address case sensitivity? Do you have some reasons you would still want to use capitalization in your email addresses?

Please let me know by leaving a comment. Thanks for reading, and caring!

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