If there are more than one possible matches, you will see a green underline on the contact name. Right-click on the green line to select the correct match.
If you see a red underline, it means that there was no match. This could indicate that the contact or e-mail address is not in your system, or that the name is spelled incorrectly. You can enter the contact into your system, or just type the e-mail address directly into the To field.
Contacts with Multiple E-mail Addresses
It is typical that people have more than one e-mail address. Once your contact's name auto-complete, you can right-click on his name to select the alternate e-mail address from the list of his e-mail addresses in the system.
If I type jeff gia into the To field, the name completes with Jeff Giasson and fills in his e-mail address. Jeff has both a personal and business address, his business address is set as default in his contact card, so that is what fills in first. If I want to send to his personal address instead (send those chain letters to him at his non-business address), I right-click on his name and select the other e-mail address from the list.
If I had typed jeff g instead, I would have gotten the green underline. There is both a Jeff Giasson and a Jeff Green. I right-click on the name to select the correct Jeff and his name and e-mail address fill in.
If I had typed jeff gii (darn sticky i key), I would have gotten the red underline indicating that there were no matches. In this case, I look again and see my typo, correct it, press Tab and see the auto-complete match up the name successfully.
Observation: I can see that typing a little bit more of Jeff's last name causes it to auto-complete without me having to right-click on it. This is going to be the same for other contacts too -- the more you type of the name, the less number of matches returned.
If I type j gia instead, I get Jeff's name and address to fill in on the first try. Any part of the first and last name, or just first name, or just last name will work for auto-complete.
The same also applies for company names if they have an e-mail address... Typing abc co gives me the e-mail address associated with ABC Company (or ABC Corporation, or just ABC Co.).