Spam is typically determined based on the content (subject and body) of the current message and the sender's reputation. For most public email providers such as Gmail, Hotmail, Yahoo!, AOL, etc., the sender's reputation is determined by previously sent messages and the sender's from address (or sender's domain).
Sending an email with the same from email address and to email address will increase the likelihood the email is flagged as spam.
Individual emails marked as spam are often delivered, but they are put into the contact's "Spam" or "Junk E-mail" folder. In some cases, email can be blocked without delivery.
Private companies and organizations often use similar metrics to determine spam; however, it can vary drastically based on anti-spam firewalls, email software, and many other factors.
To improve delivery rates, ask your contacts to add your from address (or your domain) to their "Whitelist" or "Safe Sender List".
Note: if the sender's domain is used for the sender's reputation, then all from addresses for a particular domain could be affected.