The error you're receiving is from your smtp server, which indicates that you're not authenticating with the server in the same way as before.
Are you sure you've correctly configured the smtp client? You can do so using the smtp configuration element in your .config file or at runtime programmatically.
Also, you could try using the fully qualified domain name (FQDN) of the server. For example the sender email address would be: ReceiveDelivery@sv102.internal.company.com. You can get the FQDN using the following .NET code: How to find FQDN of local machine in C#/.NET ?.
