Although it’s not advisable to convert a mail message to a string, if your objective is just to do so, you can use something called StreamReader.
Let’s say you have a string variable named strMailBody:
Using ms As New MemoryStream(Encoding.UTF8.GetBytes(strMailBody))
mail = MailMessage.Load(ms)
End Using
You can refer the following thread for more:
Hope this helps,
Best Regards.