What does the word "Ham" mean in this context?

Probably everybody knows these days whats meant with ["Spam"] when it is mentioned in combination with mail. The use od the word Ham on the other side is relatively new and sometimes confusing.

What we mean when we talk about "Ham" is more or less the "good" mail. Previously it was often called non-spam or solicited mail. Because these words are both a bit long-winded, somebody came up with the catchy word Ham.

Editorial notes:

• Can somebody please wordsmith this text a bit?
• Who was it who first introduced the word Ham, does somebody have a link?
  

Thanks, MalteStretz