<Li> RFC 5321--The Simple Mail Transfer Protocol (obsoletes RFC 821 aka STD 10, RFC 974, RFC 1869, RFC 2821, updates RFC 1123) </Li> <Li> RFC 5322--Internet Message Format (obsoletes RFC 822 aka STD 11, and RFC 2822) </Li> <Li> RFC 5504--Downgrading Mechanism for Email Address Internationalization </Li> <Li> RFC 6409--Message Submission for Mail (STD 72) (obsoletes RFC 4409, RFC 2476) </Li>

An email client need to know the of its initial smtp server