smtp client

smtp/Cargo.toml

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+[package]
+name = "smtp"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+chrono = "0.4.37"

smtp/smtp_dialog.doc

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+In a typical session, SMTP dialog works as follows:
+    - The client first establishes connection to the SMTP server.
+    - The server initiates with a greeting. This greeting indicates that the server is ready to receive commands.
+    - The client then issues its own greeting.
+    - Server responds.
+    - the client sends a command indicating who the mail is from.
+    - The server responds to indicate that the sender is accepted.
+    - The client issues anaother command, which specifies the mail recipient.
+    - The server responds indicating the recipient is accepted.
+    - The client then issues a DATA command.
+    - The server responds asking the client to proceed.
+    - The client transfers the email.
+
+Example:
+    S: 220 mail.example.com SMTP server ready
+    C: HELO mail.example.net
+    S: 250 Hello mail.example.net [192.0.2.67]
+    C: MAIL FROM:<alice@example.net>
+    S: 250 OK
+    C: RCPT TO:<bob@example.com>
+    S: 250 Accepted
+    C: DATA
+    S: 354 Enter message, ending with "." on a line by itself
+    C: Subject: Re: The Cake
+    C: Date: Fri, 03 May 2019 02:31:20 +0000
+    C:
+    C: Do NOT forget to bring the cake!
+    C: .
+    S: 250 OK
+    C: QUIT
+    S: 221 closing connection
+
+The common client commands we use are as follows:
+    HELO is used for the client to identify itself to the server.
+    MAIL is used to specify who is sending the mail.
+    RCPT is used to specify a recipient.
+    DATA is used to initiate the transfer of the actual email. This email
+    should include both headers and a body.
+    QUIT is used to end the session.
+
+The server response codes used in a successful email transfer are the
+following:
+    220: The service is ready
+    250: The requested command was accepted and completed successfully
+    354: Start sending the message
+    221: The connection is closing
+Error codes vary between providers, but they are generally in the 500 range.