Does it always fail with the same e-mail addresses, or does it appear to be
random? e.g., if you make two data files:
a. one with the records that go wrong
b. one with the records that go right

when you merge to (a), do they all still go wrong? And when you merge to
(b), do they all go right?

If that is the case, then I would have to guess that something is probably
trying to "verify" the addresses in some way (e.g. Outlook does this if I
enter my address as "Peter Jamieson" rather than pjj@whatever.

But since I do not know what is going wrong, I can only suggest that you try
the following workaround, which performs one mail merge for each record in
the data source. (Of course, if the problem is something to do with the way
that each e-mail address is processed, this will probably still fail).

e.g.
Sub ProduceOneEmailPerSourceRec()
'
' NB, needs bettor error management and doubtless other things a VBA expert
' will point out.

Dim intSourceRecord
Dim objMerge As Word.MailMerge
Dim bTerminateMerge As Boolean

' Need to set up this object as the ActiveDocument changes when the
' merge is performed. Besides, it's clearer.

Set objMerge = ActiveDocument.MailMerge
With objMerge

' If no data source has been defined, do it here using OpenDataSource.
' But if it is already defined in the document, you should not need to
define it here.

'  .OpenDataSource _
'    Name:="whatever"

' I don't use FirstRecord, LastRecord because they do not behave
' the way you expect in all data sources.

 intSourceRecord = 1
 bTerminateMerge = False

 Do Until bTerminateMerge
   .DataSource.ActiveRecord = intSourceRecord

    ' if we have gone past the end (and possibly, if there are no records)
   ' then the Activerecord will not be what we have just tried to set it to

   If .DataSource.ActiveRecord <> intSourceRecord Then
     bTerminateMerge = True
   ' the record exists
   Else
     .DataSource.FirstRecord = intSourceRecord
     .DataSource.LastRecord = intSourceRecord
     .Destination = = wdSendToEmail
     ' set up the field containing the e-mail address - use your own field
name
     .MailAddressFieldName = "eaddress"
     ' Set up the subject - make sure any field name in here has the same
     ' capitalisaiton as the name in the data source
     .MailSubject = .DataSource.DataFields("FirstName").Value & _
                            "- here's your data for 2007"
     .Execute
     intSourceRecord = intSourceRecord + 1
   End If
 Loop
End With