Sending email from webform using SMTP server - asp.net

Question

I have a ASP.NET web form for sending emails using SMTP. The hosting provider is 1and1. It worked for year. It stopped working several weeks ago. I receive the following error:
"The SMTP server requires a secure connection or the client was not authenticated. The server response was: Authentication required"
1and1 support is unable to explain the reason for the change. I would appreciate your help (see the VB.NET code below).
I tried to change the port to 465 and 587, as well, I tried to change "EnableSsl" flag to true state - nothing worked.
I wrote another code that works, but it allows to send an email with my email address in the "from" field (see the code below). But I would like to send an email with visitors' address in the "from" field.
' code that worked once
Dim mailMessage As New MailMessage()
Dim MailAddress As String
Try
'creating an instance of the MailMessage class
mailMessage.From = New MailAddress(txtEmail.Text)
'senders email address
mailMessage.To.Add("me#mysite.net")
'recipient's email address
mailMessage.Subject = "Form Submission: " & txtSubject.Text
'subject of the email message
mailMessage.IsBodyHtml = False
'message text format. Can be text or html
mailMessage.Body = txtName.Text & vbCr & txtEmail.Text
'message body
mailMessage.Priority = MailPriority.Normal
'email priority. Can be low, normal or high
Dim smtp As New SmtpClient("smtp.ionos.com")
smtp.Port = 25
smtp.EnableSsl = False
smtp.Send(mailMessage)
'using the static method "Send" of the SmtpMail class to send the mail
Response.Write("Mail sent")
Catch ex As Exception
lblError.Text = "Form processing error occurred." & vbCr & "Please send your request to: me#mysite.net"
End Try
' code that is working today, but my email address must be in the "from" field
Dim mailMessage As New MailMessage()
Dim MailAddress As String
Try
'creating an instance of the MailMessage class
mailMessage = New MailMessage()
mailMessage.From = New MailAddress("me#mysite.net")
'senders email address
mailMessage.To.Add("me#mysite.net")
'recipient's email address
mailMessage.Subject = "Form Submission: " & txtSubject.Text
'subject of the email message
mailMessage.IsBodyHtml = False
'message text format. Can be text or html
mailMessage.Body = txtName.Text & vbCr & txtEmail.Text
'message body
mailMessage.Priority = MailPriority.Normal
'email priority. Can be low, normal or high
Dim smtp As New SmtpClient'("smtp.ionos.com")
smtp.UseDefaultCredentials = False
smtp.Credentials = New Net.NetworkCredential("me#mysite.net", "MyPassword")
smtp.Host = "smtp.ionos.com"
smtp.Port = 587
smtp.EnableSsl = True
smtp.Send(mailMessage)
'using the static method "Send" of the SmtpMail class to send the mail
Response.Write("Mail sent")
Catch ex As Exception
lblError.Text = "Form processing error occurred." & vbCr & "Please send your request to: me#mysite.net"
End Try