I have to copy all the data transferred into a mail (SMTP) into a file. I don't think I have to implement a new dissector because SMTP one already exist, no?
Right.
So I think I have to complete the existing one and add a method to create a file and copy the data in. Am I in the right way to do that job?
The basic idea is for sure correct. I think you must at least add the TCP reassembly stuffs to be able to guarantee that you're playing with the entire mail and the export object GUI Feature to save to a file (the mail or its attachments). You can take a look at epan/dissectors/packet-http.c to see how to do this.
So where is defined smtp dissector?
epan/dissectors/packet-smtp.c
Regards,
Sebastien