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Using valid characters in your e-mail address

By Ian "Gizmo" Richards "Windows Secrets"

E-mail standards let you use characters other than letters and numbers in your addresses.

Unfortunately, various ISPs and webmail systems have differing rules governing which addresses are acceptable, making the whole area a bit of a mess.

E-mail address standards ignored by some ISPs

If you've ever sent an e-mail that's gone missing, or if you failed to receive a message someone sent you, it's possible that an invalid e-mail address was the culprit. The trick is in knowing a properly crafted address from an invalid one. This isn't as easy a matter as it may seem.

Allow me to demonstrate with this quick test: Which of the addresses below is invalid?


All.Geeks.are." #$%&*"@[]

From the point of view of Internet standards, the first rather innocuous-looking address is actually invalid, because it's illegal to have a period next to the @ sign. The ghastly-looking second address, by contrast, is perfectly valid, though it's unacceptable to at least one major webmail service — Hotmail — and probably others. To find out why, read on.

The allowable form of e-mail addresses is explained in an Internet Engineering Task Force document known as RFC 5322 and, to a lesser extent, in IETF document RFC 5321. These documents are pretty impenetrable, so let me translate the relevant bits into simple English for you.

An e-mail address consists of two parts separated by the @ sign. The bit to the left of the @ sign is called the local-part, and the bit to the right is called the domain. So if your e-mail address is joebloggs@hotmail.com, then "joebloggs" is the local-part and "hotmail.com" is the domain.

You may be surprised to learn that, according to the strict standard, the local-part is case-sensitive. That means joebloggs@hotmail.com is a different e-mail address from JoeBloggs@hotmail.com.

However, this part of the standard is ignored by most ISPs, webmasters, and webmail services, which usually treat the local-part as case-insensitive. This, as we shall see, is just one of several areas where practice rather than standards defines the validity of a particular e-mail address.

Another example is the use of special characters. According to the standard, the local-part can be up to 64 characters long and can consist of a mix of letters, numbers, or any of the following 20 special characters:

! # $ % & ' * + - / = ? ^ _ ` . { | } ~

These characters, except the period, may appear anywhere in the local-part of the address The period may not be used as the first or last character of the local-part nor twice in succession.

That may be what the standard says, but most e-mail providers avoid issuing addresses that contain these special characters. For example, many services allow only a single period to be used in the local-part of an address.

E-mail services often allow you to create addresses containing hyphens and underscores in the local-part, but this practice is far from universal. For example, Gmail won't allow hyphens or underscores. (Note that Gmail uses a plus sign [+] to designate throwaway addresses; more on these address extensions below.)

Likewise, the IETF standard allows up to 64 characters in the local-part, but very few e-mail services allow addresses anywhere near that long.

So we have a very strange situation: clearly, defined standards exist for e-mail address validity, yet the standards are largely ignored by webmail services, ISPs, and other e-mail providers, each of whom sets its own standards.

Real-world standards: Gmail, Yahoo, and Hotmail

I decided to find out exactly what e-mail addresses are allowed by the major webmail services: Hotmail, Gmail, and Yahoo Mail. There are actually two issues: which e-mail addresses these services will issue to users, and which addresses the services accept for outbound mail.

I was hopeful this would be neatly documented, but it's not. I had to conduct my own experiments, applying for new e-mail accounts to determine which addresses each service will accommodate.

1. Gmail. Google's Gmail will issue e-mail addresses only when the local-part is between six and 30 characters and consists of letters, numbers, and a single period. Furthermore, the first and last characters of the local-part cannot be a period. Oddly, a period within the local-part is effectively ignored, so john.smith@gmail.com is regarded by Gmail as the same address as johnsmith@gmail.com.

Once you have a Gmail address, you can expand it by adding a plus sign to the local-part and then some additional characters to create a throwaway address for signing up at questionable sites. Should that address become a spam magnet, you can create a filter that redirects to your spam folder or trash folder any mail using it. Unfortunately, many of the webforms used at major sites don't recognize plus signs in e-mail addresses.

2. Yahoo. Yahoo Mail's system for creating temporary addresses based on your primary address is more practical. This is a principal reason Scott Dunn gives Yahoo Mail the edge over competing webmail systems in his July 24, 2008, review of the services.

The local-part of a Yahoo Mail address must be between four and 32 characters and must start with a letter. It may contain letters, numbers, underscores, and one period but can't end with a period or underscore.

3. Hotmail. Microsoft's Hotmail service takes a different approach: the local-part must be between one and 38 characters long. It may contain letters, numbers, periods, hyphens, and underscores. However, the local-part must start with a letter and must not end with a period. As with Gmail and Yahoo Mail, adjacent periods are not allowed.

The conclusion is immediate. There are no universal standards; every mail system sets its own.

Only three characters are universally accepted

In the second part of my test, I tried to send an e-mail from Gmail, Yahoo Mail, and Hotmail accounts to a special e-mail address I set up at my Web site: !#$%&'*+-/=?^_`{|}~@techsupportalert.com. That's a perfectly valid address from the point of view of the Internet standard, but it's well outside what's acceptable to these three services for their own users' addresses.

Both Gmail and Yahoo Mail allowed me to send mail to this strange address. Furthermore, the e-mail arrived at that account on my site's mail server. That's good news. It means that even though Gmail and Yahoo Mail are very restrictive in what addresses they're prepared to issue to their users, they're not as strict when it comes to e-mail sent from their system. In effect, you can send e-mail to virtually any valid address from these services.

With Hotmail, it was a different story. Hotmail would not allow me to send messages to my strange-but-legal e-mail address — I got an error message as soon as I pressed the Send button. I did a little experimentation, and it looks like Hotmail won't accept any special characters in an address you're sending to, with the exception of hyphens, underscores, and periods: - _ .

If one of your friends has an address containing other special characters, you're out of luck.

I suspect Hotmail isn't the only e-mail service that won't let you send to legally valid addresses containing special characters. You can test your mail system's support for these characters by sending a test message to !#$%&'*+-/=?^_`{|}~@techsupportalert.com.

How to craft a well-formed e-mail address

There are definite standards that define which e-mail addresses are valid. However, these standards are largely ignored by mail services, which instead define their own standards for the forms of address that are acceptable.

In this somewhat chaotic situation, your best bet is to use an e-mail address that everyone accepts — the lowest common denominator.

I suggest that your address start with a letter and that the local-part consist of six or more characters — letters and numbers only. Stay away from special characters, even though the e-mail standards permit them. That includes hyphens and underscores. The use of a period is OK, provided it doesn't occur at the start or end of the local-part. Avoid using two periods, and certainly never have two periods next to each other.

To play it really safe, stick with lower-case letters.

You may get away with using a more complex address, but by following the advice in this article, you're unlikely to have your e-mail bounce due to address-validity problems. As a bonus, you're never going to have your address rejected when registering at Web sites or signing up for newsletters.


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