what are we looking into po ba? sa internal lang yung hacker or sa external? Kasi pwede deploy ng parang wormlike file na mag a act na mag phone home sa PC sa WAN eh. Or kung internal yan, nakalimutan ko yung technical term na pwedeng gamitin para mag act yung workstation niya as a decoy server eh. I think it's mac spoofing??? Oh, I forgot, the answer is yes.
Although this can be done thru internal network, if you want to intercept all email messages to your account you better be skillful enough to do a lot of hacking techniques.
Let's say you are in the same network using Outlook as your mail service. The easiest way to hack the account is to brute force the victim's account password. although this may take some time if the victim's password Complexity is strong (Combination of Symbols, Numbers, Special Characters etc.) but if it uses weak password, you'll be lucky enough to crack the password within minutes. Reliable and strong password cracker is John The Ripper. google it for the download but take precaution for possible trojans.
If you have no patience for cracking passwords, you may try Session Hijacking and Session Replay. this also can be done thru sniffing. just pray that you can find cleartext username and passwords during your capture. although, some drawback of sniffers is that they cannot sniff data if your network uses layer 2 devices or switches. This is where you can use the technique Mac Spoofing and Port Mirroring.
I have given you some ideas try researching and studying..it pays.