Delivery failure. error 601.

I have been trying to get my asp.net application to send mail dynamically retrieving the email from my data base, which when I step through the program says it grabs the address correctly, as well as i do not get an error message when a message is sent from my web page online. I have gotten a number of e-mails from the account i am trying to send from that include:

Failed Recipient: [email protected]
Reason: Remote host said: 601 Failed to connect to the recipients mail server. No MX records were found for the 'hotamil.com' domain. Attempted to send the message to the following ip's: some random IP.

Why am i getting this error? If i hard code in an e-mail address I get better luck with the e-mail getting sent.

My code for sending the e-mail is as follows.

string emailFrom = EmailAddressBox.Text;
string descriptionBody = DescriptionBox.Text;
string strTo = UserEmail;
string strFrom = "[email protected]";
string strSubject = "Your posting: '" + Title + "' has interest";
string strBody = "Reply to: " + emailFrom + ". If you hit reply without creating a new e-mail to the previous e-mail you will get no reply. Thank you." + System.Environment.NewLine +
System.Environment.NewLine + "The person who has shown interest in your posting sent the following: " + System.Environment.NewLine + descriptionBody;

MailMessage message = new MailMessage();
message.From = new MailAddress(strFrom);

message.To.Add(new MailAddress(strTo));

message.Subject = strSubject;
message.Body = strBody;

SmtpClient client = new SmtpClient();
client.Send(message);

Thank you.