[TheMightyKnighty]

I was just wondering wether I can use Wireshark, or any other packet sniffer, to see e-mails with unencrypted protocols (POP3, IMAP, SMTP) and look at the contents of that e-mail? For example seeing the contents of a PDF file being sent from one computer to another. via e-mail Also, how can I see files being sent from one device to another over a network (not using e-mail), like a file being sent from a computer to a printer?

[DramaKing]

Yes and no. Yes, it's technically possible. No, you will never see those protocols in use outside of a lab environment designed for this.

Printers, however, are (usually) easy. Just capture on port 9100 or 631. Print jobs could be sent over SMB also, possibly RDP.