Thanks for your help :)
Some code:
For Each oFile In oFolder.Files
Dim filename As String
filename = oFile.name
If filename Like "*.htm" And Not filename Like "Teampostfach*" Then
sig = oFile.OpenAsTextStream(1, 0).ReadAll
End If
Next oFile
...
outMail.HTMLBody = emailText & sig
Firstly, concatenating two HTML documents won't produce a valid HTML document. The two need to be merged, and that includes styles as well as the HTML data itself.
You also need to explicitly add signature images as attachments and modify the img tags appropriately to point to these image attachments.
You can try to display the message first (Outlook will add the signature to the empty message), and then append your HTML message body.
If using Redemption (I am its author) is an option, it exposes RDOSignature object and allows to insert any signature without displaying the message using RDOSignature.ApplyTo()
method.