The server response was: Authentication required even though authentication is provided - vb.net

Question

IONOS are not helpful. Especially as all of a sudden, a form stops sending an email.
This article explains and helps very well Sending email from webform using SMTP server
The error I get is the same: The SMTP server requires a secure connection or the client was not authenticated. The server response was: Authentication required
Here's my VB - I took on board the advice here and made the From the same as the authentication but still no luck. Originally it was objMM.From = New MailAddress(txtEmail.Text). I did also read that port 25 could work but this didn't work either. It was originally 587.
Sub btnSendFeedback_Click(sender as Object, e as EventArgs)
'Create an instance of the MailMessage class
Dim objMM as New MailMessage()
'Set the properties - send the email to the person who filled out the
'feedback form.
objMM.To.Add("receiver#receiver.com")
objMM.From = New MailAddress("website#domain.co.uk")
'Send the email in text format
objMM.IsBodyHtml = False
'Set the priority - options are High, Low, and Normal
objMM.Priority = MailPriority.Normal
'Set the subject
objMM.Subject = "Subject"
'Set the body
objMM.Body = " various long boring fields here . . . "
Dim smtp As New SmtpClient()
smtp.Host = "smtp.ionos.com"
smtp.EnableSsl = True
smtp.UseDefaultCredentials = False
smtp.Credentials = New Net.NetworkCredential("website#domain.co.uk", "passwordhere")
smtp.UseDefaultCredentials = True
smtp.Port = 587
'Send method of the SmtpMail class
smtp.Send(objMM)
End Sub
Is there anything else wrong here I wonder? IONOS are sure it is a script problem.

Answer 1

The fine manual for SmtpClient.Credentials says:
Some SMTP servers require that the client be authenticated before the server will send email on its behalf. To use your default network credentials, you can set the UseDefaultCredentials to true instead of setting [smtpClient.Credentials] property. If the UseDefaultCredentials property is set to false, then the value set in the Credentials property will be used for the credentials when connecting to the server. If the UseDefaultCredentials property is set to false and the Credentials property has not been set, then mail is sent to the server anonymously
As a result, I would expect your code to contain only one mention of UseDefaultCredentials, for purposes of setting it to False..
Unless of course, you have these same credentials configured as the default credentials in the CredentialCache.. in which case mentioning them again here is redundant