gmail's automatic address saving

I was curious what addresses are displayed in the "To:" Field in gmail and started to guess ...

are those the e-mail addresses from

• the contact-list
• the sender field of received mails (read/unread mails)
  
• the receiver field of sent mails
• a combination of sender/receiver field (all of inbox/outbox, history, recent ...)
• ...
  

after reading some discussion group threads it seemed to be the first option - contact list

but how does it work that a email-address gets into my contact-list without doing anything? ... or better - when does google save the e-mail address into the contact-list?

I started a test and

• set up a new gmail account
• wrote an e-mail from the new account to myself
• checked when the e-mail address gets into my contact-list
  

here is the result

• received email from email-address not in contact list
• e-mail address didn't appear in the "To:" field neither in the contact-list
• opened email from email-address not in contact list
• e-mail address didn't appear in the "To:" field neither in the contact-list
• replyed to email from email-address not in contact list
  
• e-mail address is saved in contact-list & appears in the "To:" field