Do not check email adresses!

You may have noticed that on some rare websites (≈ 99.999 % of internet websites), you have to provide an email address when you register. And most of the time the application running the website check if the email address you entered is valid.

But you know what? Most software developers (you know, the guys staying all day long in front of computers, the guys you call when you have a problem with your computer because they have some magical power that allows them to fix your problems) don't know what a valid email address is!

Why do I say such things? Because each time I register on a website, it complains that I should enter a valid email address… but my email address is valid actually! What I'm complaining about is not that they check email addresses of course, I'm complaining about the fact they are rejecting valid ones. Like for an email spam filter, you don't care that some spam emails still reach your inbox, but you hate when a valid email is dropped because it was considered spam.

Unfortunately, most email address filters that have been written are rejecting a lot of valid email addresses. Why? Because checking that an email address has a valid format is very difficult. Have a look at this summary of RFC's email address format, you will understand what I mean.

So, if you are a software developer and you have to check the validity of email addresses, don't bother much checking the email address format (or the strict minimum like "there is a '@' and a '.'") because even an email address that have a valid format may be invalid (in the way that the corresponding email account does not exists).

For registration process it's easily solved by asking the user to click on a confirmation link you send them to the given email address. Only then you will know that the email address is really valid.