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What can you put in your email address?

A simple question – what characters can you put in an email address? Like many things to do with the Internet the answer isn’t as simple or direct as you might think. We’ll look at what makes a valid email address, both in theory and in practice.

by Email Essentials

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A simple question - what characters can you put in an email address? Like many things to do with the Internet the answer isn't as simple or direct as you might think.

We'll look at what makes a valid email address, both in theory and in practice.

For daily use you don't need to know. If you simply copy the email address as you're given it, you should be OK.

But knowing a bit more about email addressing can help you work out if an email address is incorrect or identify why it doesn't work. Developers and programmers might be surprised to discover that their carefully written web pages or code aren't entirely correct.

Which ones are correct?

Which of these email address (all fake) is formatted correctly?

  • mailto:Fred
  • Fred*
  • FredO'
  • Pa$

The answer is that they are all strictly valid though they might not be useable in practice.

Knowing a bit more about email addresses may well interested many readers of 'Email Essentials' who, like us, get intrigued by these details.  In keeping with the 'Essentials' part of our name, this is by no means a comprehensive look at email address formatting.  We've provided links to the various RFC specification documents if you're interested in the minutiae.

Local Address @ Domain

There are two parts to an email address - the 'Local Part' and the Domain - which are separated by the famous @ symbol.

For example has 'fred' as the local part and '' as the domain.

Historical note: back at the start of the internet, Ray Tomlinson developed the first simple email system to work between computers. He's the guy who chose the @ symbol to separate the name and domain name.

The two parts of an email address have different rules about what is permitted. Domains are much more limited than local parts.

Domain rules

A domain name can contain letters, digits and hyphens only, up to a maximum of 255 characters.

Each part of a domain name is separated by the .(aka dot, fullstop or period).

Domain naming is a whole article on its own - suffice it to say what we're used to domains like .com .edu etc but there are also country domain suffixes (Top Level Domains TLD's) like .au .uk and .us right down to obscure ones (for the usually uninhabited Heard and McDonald Islands). See here for a full list.

There's no consistency about domain suffixes. The commercial domain name is a good example. In the US it's .com as we all know.Australia clones that for but the UK uses and New Zealand follows suit with .

That's pretty straight-forward, the surprises come when you look at the part before the @ symbol …

Article posted: Friday, 15 September 2006

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