Email validation on textField in iOS

Use NSPredicate and Regex:

- (BOOL)validateEmailWithString:(NSString*)email
{
    NSString *emailRegex = @"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}"; 
    NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegex]; 
    return [emailTest evaluateWithObject:email];
}

For a bunch of emails separated by a comma:

- (NSMutableArray*)validateEmailWithString:(NSString*)emails
{
    NSMutableArray *validEmails = [[NSMutableArray alloc] init];
    NSArray *emailArray = [emails componentsSeparatedByString:@","];
    for (NSString *email in emailArray)
    {
        NSString *emailRegex = @"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}"; 
        NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegex]; 
        if ([emailTest evaluateWithObject:email])
            [validEmails addObject:email];
    }
    return [validEmails autorelease];
}

Edited Answer: (It also validates extra dots )

- (BOOL)validateEmailWithString:(NSString*)checkString
{
    BOOL stricterFilter = NO; // Discussion http://blog.logichigh.com/2010/09/02/validating-an-e-mail-address/
    NSString *stricterFilterString = @"[A-Z0-9a-z\\._%+-]+@([A-Za-z0-9-]+\\.)+[A-Za-z]{2,4}";
    NSString *laxString = @".+@([A-Za-z0-9-]+\\.)+[A-Za-z]{2}[A-Za-z]*";
    NSString *emailRegex = stricterFilter ? stricterFilterString : laxString;
    NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegex];
    return [emailTest evaluateWithObject:checkString];
}