Maintaining or building a good relationship through email is not as easy as building bonds by physically being together. But with a little attention, a good "virtual" relationship can easily be cultivated.
In everyday emails, most people don't want to wade through lengthy paragraphs in a foreign language to decipher the point. On the other hand, Japanese do like to take a softer approach, especially when it has been a little while since the previous communication. This softer introductory sentence or paragraph is often a general comment about the weather and an inquiry after the other person's health.
For example, during summer you could say, "I have heard that the summer's heat continues in Japan, but I hope you are tolerating it in good health," or, "Everyday it gets hotter here. How are you doing?" In October you could write, "The maple leaves must be beautiful there now. I trust you are in good health?" and in November, "As each day grows colder, I hope you are still staying well."
Since it's not automatic for Americans to start this way, many find it helpful to write their email first, and then go back and write the introduction. Very often it's a matter of moving up front the sentences you might put at the close of your email.
Just two sentences can help to communicate: "I care about you as a person and not just as a means to make money or get my work done," and they pave the way for the recipient to look at the rest of the email with that in mind. Whether virtual or in person, letting the other person know you care is a good way to build and reinforce the relationship.