Deep Learning approach for Signature extraction from emails
Detecting a signature in an email is more like a detection problem in an image rather than a natural language processing (NLP) problem. It does not particularly matter what the signature says, you simply want to be able to detect it. 

In such a case, detection in images is currently dominated by convolutional neural networks (CNN). This is a very powerful technique which has proved to be the best for a multitude of image-based tasks. However, it requires a very large dataset. So you will need to collect a lot of varied examples and annotate them yourself. 

The output of the CNN can provide either a binary output 0/1 determining if a signature exists. Or it can provide an output which is the coordinates of the center of the signature (X, Y coordinates) such that you can know where it is placed in the document.