Topic: SMTP or phpmailer
php mail function is becoming unreliable this days, not to mention gmail and yahoo spam filter doesn't like emails sent this way. Also, if you are on a shared host account, you can experience a few hours delay in sending out email.
A better alternative is phpmailer or even better SMPT.
If phpmailer is used, it will be very useful to have the sender as the login user account. This way Salesman Bob is sending quote email to Company A, so that if Company A have questions, it will call Salesman Bob.