Thank you for your post.
At this time, FileMaker Pro is unable to access the signature information in the email client. FileMaker Pro sends the information to the email client where the email client takes over and sends it out. I will forward your request to our Development and Software Quality Assurance (Testing) department to consider accessing the signature preferences in Apple Mail. I would also suggest entering this information into our Feature Suggestion web form at:
Although I could copy your post and paste it into the web form, there are additional questions asked that only you can answer.
In the mean time, please have the clients include their signature in the message that is being sent from FileMaker Pro.