Strange .NET Mail behaviour with Disposables - asp.net

Question

Recently been going through a bunch of code tidying up and "improving" things, I noticed this bit of code used for sending emails:
Dim SmtpServer As New SmtpClient() With {
.Port = 25,
.Host = Email.Hostname,
.Credentials = New NetworkCredential() With {
.UserName = Email.UserName,
.Password = Email.Password
}
}
Dim mail As New MailMessage() With {
.From = Email.Sender,
.Subject = Email.Subject,
.IsBodyHtml = True,
.Body = Email.WrappedHtml
}
If Not Email.Attachments Is Nothing Then
For Each a In Email.Attachments
mail.Attachments.Add(a)
Next
End If
mail.To.Add(New MailAddress(Email.Recipient.Email, Email.Recipient.Name))
Call SmtpServer.Send(mail)
SmtpServer.Dispose()
mail.Dispose()
SmtpServer = Nothing
mail = Nothing
In that example the Email Object is a Class containing basic email information, recipients, sender, attachments etc.
The code above works perfectly....
But, I noticed that SmtpServer and MailMessage are both Disposable objects, and it would be better practice to wrap them in Using statements, thus:
Using SmtpServer As New SmtpClient() With {
.Port = 25,
.Host = Email.Hostname,
.Credentials = New NetworkCredential() With {
.UserName = Email.UserName,
.Password = Email.Password
}
}
Using mail As New MailMessage() With {
.From = Email.Sender,
.Subject = Email.Subject,
.IsBodyHtml = True,
.Body = Email.WrappedHtml
}
If Not Email.Attachments Is Nothing Then
For Each a In Email.Attachments
mail.Attachments.Add(a)
Next
End If
mail.To.Add(New MailAddress(Email.Recipient.Email, Email.Recipient.Name))
Call SmtpServer.Send(mail)
End Using
End Using
However... when I do that, and send more than a couple of emails in quick succession I get:
System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed. at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean read430) at System.Net.Mail.SmtpReplyReaderFactory.Read430s(SmtpReplyReader caller, Boolean one430) at System.Net.Mail.SmtpReplyReaderFactory.Read430(SmtpReplyReader caller) at System.Net.Mail.CheckCommand.Send(SmtpConnection conn, String& response) at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, MailAddress from, Boolean allowUnicode) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message)
It appears that the Using Statements are causing the connection to close too early....
Anyone else had similar experience/tips on solution?
I've reverted to the old code, but it is better practice to wrap in using statements, right?