Just WHAT EXACTLY is a spam?

I have asked a similar question before because I was annoyed receiving mails without my own address. All the answers were from the standpoint of the receiver. I am beginning to live with it let alone for the possibility of it being a vehicle for viroses they say. Now, I am annoyed with my mails that could not go through. Allegedly it is a spam. This is quite devastating in my part thus I want to know so that I CAN NOT and WILL NOT create a spam. The way I sense it, it is in the form. At least, that is what the notice says. I am certain that I am not using indecent language nor selling or promoting anything with out appropriate charge. Therefore, what is the prescribed and standard form? I can not afford the time for trial and error way of finding out. What is the criteria on which the spamguard was based?