MailSubject Property
Returns or sets the subject line used when the mail merge destination is electronic mail. Read/write String.
expression.MailSubject
expression Required. An expression that returns a MailMerge object.
Example
This example merges the document named "Offer.doc" with its attached data document. The results are sent to the e-mail addresses stored in the EmailNames merge field, and the subject of the mail message is "Amazing Offer."
With Documents("Offer.doc").MailMerge
.MailAddressFieldName = "EmailNames"
.MailSubject = "Amazing Offer"
.Destination = wdSendToEmail
.Execute
End With