Network Programming in .NET

If this appears in the Mail enable logs:

Authenticating User: a@b.com using Authentication Provider Credentials
04/01/10 12:43:19    ME-I0129: Account b.com Mailbox a@b.com prevented from sending email as a@c.com
04/01/10 12:43:23    ME-I0070: (recv) socket [780] was gracefully closed during [MAIL] command by the remote client xxx.xxx.xxx.xxx
04/01/10 12:43:23    ME-I0074: [780] (Debug) end of conversation

Fix:

Start > Run > mailenable.msc

Open Servers / Localhost / Connectors / SMTP

Right Click

Security

Uncheck "Authenticated senders must use valid sender address"

Restart "MailEnable SMTP Connector" from Control Panel / Administrative tools / Services