You didn't post your code, however, the following should change the sending account to then named account of ALL the mail items in the default outbox
Sub ChangeAccount()Dim oMail As MailItem
Dim oAccount As Account
Dim i As Integer
Const strAcc As String = "display name of account"
For Each oAccount In Application.Session.Accounts
If oAccount.DisplayName = strAcc Then
For i = Session.GetDefaultFolder(olFolderOutbox).Items.Count To 1 Step -1
Set oMail = Session.GetDefaultFolder(olFolderOutbox).Items(i)
With oMail
.SendUsingAccount = oAccount
.Send
End With
Next i
Exit For
End If
Next
Set oAccount = Nothing
Set oMail = Nothing
lbl_Exit:
Exit Sub
End Sub
Or you could use https://www.gmayor.com/ManyToOne.htm in one to one mode and merge to the account of your choice.