Secondly and more importantly; even if the email matches a perfect regular expression, there are no guarantees that the email address belongs to the user, or even exists.

With all that said I still believe live javascript-based email address checking using regular expressions is a good idea. You can use it to prevent users from doing unnecessary typos. You don't have to force them to pass the validation, just let them know if you suspect the email to be faulty. Try the demo, created with the Jquery plugin Valid8, above and you will see what I mean.

/^[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+(aero|asia|biz|cat|com|coop|edu|gov|info|int|jobs|mil|mobi|museum|name|net|org|pro|tel|travel.ac|ad|ae|af|ag|ai|al|am|an|ao|aq|ar|as|at|au|aw|ax|az|ba|bb|bd|be|bf|bg|bh|bi|bj|bm|bn|bo|br|bs|bt|bv|bw|by|bz|ca|cc|cd|cf|cg|ch|ci|ck|cl|cm|cn|co|cr|cu|cv|cx|cy|cz|de|dj|dk|dm|do|dz|ec|ee|eg|er|es|et|eu|fi|fj|fk|fm|.fo|fr|ga|gb|gd|ge|gf|gg|gh|gi|gl|gm|gn|gp|gq|gr|gs|gt|gu|gw|gy|hk|hm|hn|hr|ht|hu|id|ie|.il|im|in|io|iq|ir|is|it|je|jm|jo|jp|ke|kg|kh|ki|km|kn|kp|kr|kw|ky|kz|la|lb|lc|li|lk|lr|ls|lt|lu|lv|ly|ma|mc|md|me|mg|mh|mk|ml|mm|mn|mo|mp|mq|mr|ms|mt|mu|mv|mw|mx|my|mz|na|nc|ne|nf|ng|ni|nl|no|np|nr|nu|nz|om|pa|pe|pf|pg|ph|pk|pl|pm|pn|pr|ps|pt|pw|py|qa|re|ro|rs|ru|rw|sa|sb|sc|sd|se|sg|sh|si|sj|sk|sl|sm|sn|so|sr|st|su|sv|sy|sz|tc|td|tf|tg|th|tj|tk|tl|tm|tn|to|tp|tr|tt|tv|tw|tz|ua|ug|uk|us|uy|uz|va|vc|ve|vg|vi|vn|vu|wf|ws|ye|yt|yu|za|zm|zw)\b$/

Above is the regular expression I use. It is heavily based on a regexp from regular-expressions.info

How do I validate email addresses if not with regular expressions?

Sending an email activation link is a perfect way of validating an email address. You email a link the user have to click on to complete the registration. The link should be disposable and often look like this: http://www.unwrongest.com/signup/?guid=8373629375284563

Never trust client-side validation

Lastly. Remember that you can never, ever, trust client-side javascript validation. It is very easy to tamper with. All validation you do on the client-side has to be done again on the server-side. Client-side validation should be mainly for the users sake.