Why do I get the error message ‘Sender address rejected: need fully-qualified address ‘

This is happening because your SmtpMail.SmtpServer is rejecting addresses.
To resolve this make sure

• all email addresses specified at MailMessage.To, MailMessage.Cc, MailMessage.Bcc and MailMessage.From are valid email addresses
• you have permissions to relay through the server.
• MailMessage.From has permissions to relay through the server