Any email you send/receive from the company is the property of the company and therefore as an IT employee you're entitled to ensure the company's assets are being used in accordance company policy. Using company equipment for personal use is often allowed but within "reasonable use". In an ideal world you should report this email as abuse of company property and you should be able to remain anoymous as the problem should then be dealt with by the HR dept and the offender's manager as they see fit. It's not your job to pass moral judgements, just act within the bounds of company rules and policy.
Suppose the emails contained evidence of some crime that was later committed? In the financial world everyone, including IT staff, have to take mandatory, annual anti-money laundering training and certification. If you know about potential ML then you're bound by law to report it to the correct company officer else you risk a spell in clinky.