Like any data, an e-mail address that you accept from a user cannot just be accepted at face value, it needs to be validated. It's non-trivial to do this, though -- not only do you have to make sure that it is formatted correctly (and this is harder than it first appears, as the rules for what constitute a valid e-mail address aren't simple), you also have to make sure that the e-mail address will actually receive mail.
The EmailAddressValidator class tries, as much as possible without actually delivering an e-mail, to validate that an e-mail address is valid and will accept e-mail. It does this in three stages:
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Ensure that the e-mail address is correctly formatted, using a regular expression derived from the ABNF specified in RFC2822 for maximum correctness. This takes very little resources.
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Lookup MX records or A records for the domain part of the e-mail address, to make sure that there's someone who might be willing to receive mail. This lookup requires network access and take a little bit of time, but can catch a lot of simple typos.
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Make an attempt to deliver an e-mail to the address, stopping just before sending the message body. This is the most accurate way to discover whether an e-mail will be delivered, although it is still not 100% accurate (for example, Exchange servers will accept all mail at SMTP time and then generate a bounce, so an SMTP-time test won't tell you anything useful there). However, most e-mail services will do the right thing, and it is the only way to detect a typo'd local part. It does take a little while to perform this lookup, though.
See the rdoc/ri documentation for full information on how to use this class,
or just look in lib/email_address_validator/email_address_validator.rb.