I also must disagree with the 'snooping is just not a good idea.'
If we have trust in someone, there is no need. If there is distrust, it is important and perfectly acceptable to take whatever legal steps necessary to protect ourselves emotionally, financially, and physically.
Cybercheating is, in my opinion, every bit as wrong and a boundry violation as having sex with another. Both are selfish, disrespectful acts which harm an innocent third party.
When someone cheats, they hold the responsibility for breaking the trust. Because of their actions, the injured party is under no obligation for further extension of trust. Definitely not unless and until the cheater comes clean, accepts responsibility, ends all contact with the other(s), makes a real, long-term effort at change, and works on their serious issues.
In my job, I have been in the sad position of having to uncover evidence of infidelity, harassment, etc. for employees and others caught using workplace equipment for this vile behavior.
These people make co-workers, colleagues, and mutual friends unwilling participants in the drama, and they also take on many risks that are not theirs. These behaviors rarely get better. Sometimes, they increase in frequency & type of inappropriate behaviors including stalking, blackmail, sexual harassment, etc.
Even when caught, many say 'it didn't bother anyone.' Most often there are innocent bystanders who are burdened with negative feelings. Many 'mutual friends' know but do not know what to do. Some are afraid. It is far from 'just some email.'
Healthy, mature adults NEVER engage in such behavior. The ones who do often are caught through 'just an email...' that is often discovered to be just the tip of the iceberg hiding a tremendous amount of inappropriate behavior uncovered once investigated.
We never advise people to just ignore it and hope for the best. This behavior is sign of trouble - and the sooner it is addressed, the better.
People with any history of such behavior never make it past our first application review when applying for employment. They're just too great of a risk to trust them with our technology, our staff, and our public reputation. One instance of such harmful behavior can be a lawsuit, an investigation, a career ender, or worse.
My advice is always, trust your feelings, and get as much info as you can. Make a plan using good suggestions from trustworthy people.
Those who try to minimize this behavior have not experienced the full repercussions of this, either personally or in the workplace. But it is not them, but you, Darcy, who is the one who has to live with the consequences.
If that person receiving those emails decides to press stalking charges, or sues for sexual harassment, it will be your life and finances which are thrown into chaos. If he is stalking, or sending these to a minor, or to a person with an unhinged partner who'll come looking for him with a weapon, this can be a very dangerous situation indeed.
With so much risk, treating this as a possible serious threat to your quality of life and emotional stability is an appropriate response. You do not have to feel guilty for protecting yourself. If he truly loves you, he'll understand and work it out...and ensure i never happens again.