Simple Mail Transfer Protocol (SMTP) is the protocol that allows the i5/OS® operating system to send and receive e-mail.
SMTP is essentially the end-to-end delivery of mail from one mail server to another. There is a direct connection between an SMTP sender (the client) and the destination SMTP receiver (the server). The SMTP client keeps the mail at the sender until it transmits and copies it successfully to the SMTP receiver (server).
SMTP on the i5/OS operating system supports the distribution of notes, messages, and ASCII text documents. SMTP can support formats other than plain text by using the Multipurpose Internet Mail Extensions (MIME) protocol. MIME is the Internet standard for sending mail with headers that describe the contents of the mail messages to the receiving client. These messages can contain video, audio, or binary parts.
In order for e-mail to reach its destination, SMTP must be able to deliver it to both the correct host and user ID that resides on that host. Suppose that mail is sent to bobsmith@mycompany.com.
First, SMTP checks to see if the e-mail addressee (bobsmith) is a user on the local server. If SMTP determines that it is not, SMTP forwards the e-mail to the next host server. The next host might or might not be the final host. SMTP determines the name of the host from addressing information that is found in the SMTP protocol.
SMTP then resolves the host's address by using either the domain name server or the local host table. The host name is what people use as a part of their e-mail account (mycompany.com); the IP address is what SMTP uses to find the correct mail server to send mail to (192.1.1.10). See the DNS topic for complete information about DNS.
These topics relate DNS to SMTP:
For inbound e-mail, the SMTP server first converts the destination host name into an Internet Protocol (IP) address. Because of the aliasing function, the server can have several host names. Therefore, the SMTP server uses the sockets interface to determine if the IP address is one of those used by the interfaces for the local host.
Related concepts
DNS Mail and Mail Exchanger records