Unusual error in Clients website

Unusual error in Clients website

I just created an account on my client's website (which I build for him).
I have tested this website extensively and could not find any problems - 2
years ago. I've since tested it to make sure it's all running smoothly
every month since the website was released.
I just created a new account on his website just now, and I get this error:
Server Error in '/' Application.
Mailbox unavailable. The server response was: <me@myemail.com> No such
user here
Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information
about the error and where it originated in the code.
Exception Details: System.Net.Mail.SmtpFailedRecipientException: Mailbox
unavailable. The server response was: <me@myemail.com> No such user here
Source Error:
Line 62: HttpUtility.UrlEncode(token));
Line 63:
Line 64: WebMail.Send(
Line 65: email,
Line 66: "Please confirm your Account.",
Source File:
d:\HostingSpaces\website\website.com.au\wwwroot\Protected\Account\Register.cshtml
Line: 64
Stack Trace:
[SmtpFailedRecipientException: Mailbox unavailable. The server response
was: <me@myemail.com> No such user here]
System.Net.Mail.SmtpTransport.SendMail(MailAddress sender,
MailAddressCollection recipients, String deliveryNotify,
SmtpFailedRecipientException& exception) +1229659
System.Net.Mail.SmtpClient.Send(MailMessage message) +1772
System.Web.Helpers.WebMail.Send(String to, String subject, String body,
String from, String cc, IEnumerable`1 filesToAttach, Boolean
isBodyHtml, IEnumerable`1 additionalHeaders, String bcc, String
contentEncoding, String headerEncoding, String priority, String
replyTo) +686
ASP._Page_Protected_Account_Register_cshtml.Execute() in
d:\HostingSpaces\website\website.com.au\wwwroot\Protected\Account\Register.cshtml:64
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +208
System.Web.WebPages.WebPage.ExecutePageHierarchy(IEnumerable`1
executors) +68
System.Web.WebPages.WebPage.ExecutePageHierarchy() +123
System.Web.WebPages.StartPage.RunPage() +19
System.Web.WebPages.StartPage.ExecutePageHierarchy() +67
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext
pageContext, TextWriter writer, WebPageRenderingBase startPage) +78
System.Web.WebPages.WebPageHttpHandler.ProcessRequestInternal(HttpContextBase
httpContext) +121
Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET
Version:4.0.30319.272
What the??? The email address listed down there is the email address I
used to create the account. After getting this error, I then logged into
that email account online - and it's there - with all my mails. So the
user does exist. So then why does WebPages framework thing that this user
does not exist when trying to send an email confirmation to it?
The email variable you see up there in the code - it's value is listed in
the exception itself.