Email - Foundation

MTA (Mail Transfer Agent)
MUA (Mail User Agent)
MDA (Mail Delivery Agent)

SMTP (Simple Mail Transfer Protocol) – Send
POP (Post Office Protocol): POP2, POP3 – Receive
IMAP (Internet Message Access Protocol) – Send and receive

MUA --SMTP/IMAP--> MTA --SMTP--> ... -> MTA --SMTP--> MDA --POP/IMAP--> MUA

postfix

myhostname = smtp.johnsonlau.net
mydomain = johnsonlau.net
# SMTP 25 listen on all interfaces
inet_interfaces = all
# SMTP 25 listen on 127.0.0.1
inet_interfaces = 127.0.0.1

cat file | sendmail -f {sender-email} -F {sender-name} {receiver-email}
sendmail -f {sender-email} -F {sender-name} {receiver-email}
> subject: {this is subject}
> {this is body}
> .

Reference1 Reference2