Guys, I have to interject. This is a terrible idea:
This is horrible practice from the perspective of a mail server. Too many illegitimate email addresses and you will start getting more "soft bounces" (a polite way to say, "We're not delivering this") when attempting to send mail. ISP's keep score for how accurate a mail server is when delivering mail, attempting to sift out spammers. When your score dips to a certain level, ISP's stop cooperating with you and basically label you as a "dubious/bad actor".
For a small-scale operation, perhaps this is okay. For anything larger or more vital, I wouldn't trust this method of operation, as it will at some point result in emails to legitimate addresses not being delivered. That's simply unacceptable for many applications.
If you get an incorrect address, it's because the user made a mistake. Chances are, most of those mistakes will still be RFC-compliant addresses (wrong or missing letters, for example). I find it hard to believe that people inputting addresses without @s or with unicode characters would become a real problem.
Frankly, your lack of belief is likely due to a lack of experience with user submitted forms with email addresses. It's VERY common for users to simply type the wrong data into a particular textarea. If you do no validation you will get things like the person's name, street address, or other confused mixups.
Anyone who deals with forms of this nature will have seen this firsthand, and with enough frequency to cause trouble with mail relay as the person above has described. It's a real problem.
This is horrible practice from the perspective of a mail server. Too many illegitimate email addresses and you will start getting more "soft bounces" (a polite way to say, "We're not delivering this") when attempting to send mail. ISP's keep score for how accurate a mail server is when delivering mail, attempting to sift out spammers. When your score dips to a certain level, ISP's stop cooperating with you and basically label you as a "dubious/bad actor".
For a small-scale operation, perhaps this is okay. For anything larger or more vital, I wouldn't trust this method of operation, as it will at some point result in emails to legitimate addresses not being delivered. That's simply unacceptable for many applications.