Gets the index of the data row used for mail merge of this message.
The index of the data row in MergeTable data table used for mail merge of this message, or zero if this mail message was not composed as a merge of database data with mail template.
For mail merge operations, e.MergeTable[e.MergeRowIndex] (in C# syntax, assuming e is an instance of SmtpMessageSentEventArgs class) returns DataRow used for mail merge of the message which could not be sent.
SmtpMessageNotSentEventArgs Class | MailBee.SmtpMail Namespace