Whoah! Heavy issue here!
I don't want to step on anyone's toes, or be the cause of a domestic dispute; so what I provide here is for informational purposes only.
Since you asked specifically regarding the abilities of Little Snitch, if a LS user opens the LS Network Monitor, he or she will see a list of incoming and outgoing data on the local network, such as your wi-fi network.
By right-clicking a domain name or IP address that is listed to the right of a particular app -- say, the Mail app for example -- he will see an option to capture the data packets for that particular app.
Once the Little Snitch user determines that he or she has saved enough data packets, he can then terminate the capture, and a ".pcap" file will be saved in his or her "Downloads" folder.
The ".pcap" file can be opened and read with such apps as WireShark and Cocoa Packet Analyzer.
Thus, if a person is skilled and knows what they are doing, yes, they can read email that is flowing through a local network in this fashion.
In fact, that is precisely what the U.S. government's formerly named "Carnivore" packet sniffer does as well. They pressure ISPs and the like into attaching the Carnivore packet sniffer system to their networks, in order to sniff the data traffic of anyone they please, sometimes without the necessary authority or legal warrants to do so. Thank you, Patriot Act, 9/11, George W. Bush, etc.
Please note, however, that there are other ways to sniff data packets, even without using Little Snitch.
As I already mentioned, WireShark and Cocoa Packet Analyzer are two tools which are widely used by IT folks to analyze and troubleshoot their networks.
However, someone on your local wi-fi network doesn't necessarily have to use Little Snitch, WireShark Cocoa Packet Analyzer, etc., in order to see what you are typing in your emails.
For example, someone can clandestinely install and enable Apple Remote Desktop, Timbuktu, or similar program on your computer, without you even knowing about it. By doing so, depending on the settings, they can not only see everything that you do on your computer, but if they set it up properly, they can even control your computer.
Furthermore, if someone on your local network knows your account name and admin password, they can connect to your machine, and do quite a bit, as if they are you.
For example, they can go to the Mail app support folder in your user library, copy the mboxes to their own machine, and then read all incoming and outgoing mail, if they have the right tools. You don't have to be a very advanced Mac user to do this.
The simple truth of the matter is that despite what a lot of people continue to erroneously believe, there is very little, if any, privacy on the Internet. A persistent and skilled hacker, government agency, husband, wife, boyfriend, girlfriend, curious person, etc., can eventually access what they want, if they persevere, across the Internet, or on a local LAN, such as your wi-fi network.
If large conglomerates, government agencies, universities, credit card companies, online businesses, etc., are not immune to being hacked, and their data stolen, isn't it kind of foolish to believe that us little guys on the bottom somehow remain immune?
In light of this, figuring out how to read someone else's email is really small potatoes by comparison.
But is YOUR husband doing this? Obviously, you better have some solid, clear proof before you go accusing him.
Regardless, it looks to me like you two really need to sit down and have a serious talk, because there is obviously a trust issue here.
Let me conclude by stating that I am not very knowledgeable in this area, so my information may not be 100% accurate, but I do believe that it is close to accurate.
I am sure that the Objective Development team, and other more experienced Little Snitch users here can correct anything mistaken which I may have stated, and add to it, if they so choose.