SMTP is an Application layer protocol (RFC 821,1982). Being a completely ASCII text based communication protocol, any binary or not text based attachments must be encoded before it can be sent using SMTP.
To allow users to send rich content is MIME (Multipurpose Internet Mail Extension) was introduced. MIME is an Internet standard that extends the format of email to support. Which allows for:

•    Text in character sets other than ASCII (Such as German, Japanese, Chinese and other high ASCII and double byte languages)
•    Non-text attachments
•    Message bodies with multiple parts
•    Header information in non-ASCII character sets

SMTP Response Codes

SMTP Commands

The SMTP message envelope refers to the sender and recipient addresses used within the SMTP transaction. These can be different from the addresses within the message headers.

DNS & SMTP

SMTP relies on DNS records to determine which address will deliver mail for the subsequent domain.
Below are the various DNS records involved with SMTP mail transfer,

Registering the DNS

In order to register your device (mail exchange, spam filter etc) with DNS you need to do the following,

1. Create A record that maps the host name of the appliance to an IP address
2. Create MX record that maps your public domain to the appliances host name
3. Specify a priority fo the MX record to advertise your appliance as a primary or backup Mail Exchange

The MX Record

The MX record is the main DNS record that is associated with SMTP. As mentioned previously the MX record contains the Mail Exchanges for the necessary domain.

C:\Documents and Settings\administrator>nslookup
Default Server:  dnsserver.myisp.com
Address:  8.8.8.8
> set type=mx
> bbc.com

Non-authoritative answer:
bbc.com MX preference = 20, mail exchanger = cluster8a.eu.messagelabs.com
bbc.com MX preference = 10, mail exchanger = cluster8.eu.messagelabs.com

As you can see from the output of a queried MX record for bbc.com, we get the A records of 2 mail exchanges.  The preference number dictates which Mail Server the your mail will be sent to. This is based on Lower the value = higher the priority.

This allows for you to DNS round robin between equal cost Mail Servers and in turn equally load balance the SMTP traffic between your Mail Servers.
A common scenario is that spammers will try and connect (and relay) their spam though MX records with the highest preference number. This is due to the general thought being that companies will only have spam filter systems deployed upon their primary Mail exchangers (Mail Servers).