How to know a domain is bad before calling SmtpClient.Send()?

Question

I have code that sends emails to users from a signup webpage. Occasionally, users will mistype an email address and the address ends up having a bad domain.

I would love to be able to check to see if the domain is bad BEFORE calling the System.Net.Mail.SmtpClient.Send() method since this would lead to a better user experience. I think using AJAX to tell the user right away their email won't send without having to programmatically send an email before finding out it is bad is much more elegant.

I currently handle the error just fine so please don't discuss how to handle errors. The error I get is: "Mailbox unavailable. The server response was: : Recipient address rejected: Domain not found"

Does anyone know of a way that would use similar (or the same exact) code to verify if a domain is bad or not before calling the Send() method?

Answer 1

Here is a good article on it: http://www.codeproject.com/KB/validation/Valid_Email_Addresses.aspx

In my experience I just use a regex validator to verify format and use a confirmation type of system to ensure it is a valid address.

UPDATE: That article is old, so I would check to ensure not deprecated.

Answer 2

Perhaps use System.Net.Dns.GetHostByName(hostname) and determine if it comes back with something valid?

Although MSDN is telling me that that method is deprecated...

Maybe System.Net.Dns.GetHostEntry(hostname) is it's predecessor?

Answer 3

The most reliable way to make sure a mail server is actually configured for a given domain is to query the MX record for that domain.

Answer 4

Try something like this:

string email = "[email protected]"
MailAddress ma = new MailAddress(email); // Throws exception if email address is incorrectly formatted
System.Net.Dns.GetHostEntry(ma.Host); // Throws exception if host is invalid