Role of MTA:
- receives email from the client's MUA
- passes email to the MDA for final delivery
- uses SMTP to route email between servers
See more: http://en.wikipedia.org/wiki/Message_transfer_agent
See this links for sending multiple emails:
- Sending mass email using PHP
- Sending bulk email in PHP
Update:
A message transfer agent(MTA) receives mail from either another MTA, a mail submission agent (MSA), or a mail user agent (MUA). The transmission details are specified by the Simple Mail Transfer Protocol (SMTP). When a recipient mailbox of a message is not hosted locally, the message is relayed, that is, forwarded to another MTA. Every time an MTA receives an email message, it adds a Received trace header field to the top of the header of the message, thereby building a sequential record of MTAs handling the message. The process of choosing a target MTA for the next hop is also described in SMTP, but can usually be overridden by configuring the MTA software with specific routes.
A MTA works in the background, while the user usually interacts directly with a mail user agent. One may distinguish initial submission as first passing through an MSA – port 587 is used for communication between an MUA and an MSA while port 25 is used for communication between MTAs, or from an MSA to an MTA; this distinction is first made in RFC 2476.