If you are using Outlook functionality then Outlook needs to be open while it is in use. The following will open outlook and create the message, retaining the signature.
Note the comment at the top of the macro re required code
Sub Send_As_Plain_EMail()'Graham Mayor - http://www.gmayor.com - Last updated - 14 Jul 2017
'Requires the code from http://www.rondebruin.nl/win/s1/outlook/openclose.htm
'to either retrieve an open instance of Outlook or open Outlook if it is closed.
Dim bStarted As Boolean
Dim olApp As Object
Dim oItem As Object
Dim objDoc As Object
Dim strRecipient As String
Dim strSubject As String
Dim oRng As Object
strRecipient = "someone@somewhere.com"
strSubject = "This is the message subject"
ActiveDocument.Range.Copy
Set olApp = OutlookApp()
Set oItem = olApp.CreateItem(0)
With oItem
.BodyFormat = 1 'Plain text (2 for html)
.Display 'This line is required
Set objDoc = .GetInspector.WordEditor
Set oRng = objDoc.Range
oRng.Collapse 1
oRng.Text = "This is the message"
.To = strRecipient
.Subject = strSubject
'.Send 'Restore after testing
End With
lbl_Exit:
Set oItem = Nothing
Set olApp = Nothing
Set objDoc = Nothing
Set oRng = Nothing
Exit Sub
End Sub