Email algorithm

Hello! Basically, what's the algorithm for email specifically an email client?

Here's what I want to do:
1. The email client will render html content but will not run executable scripts.

2. If the user's storage space has exceeded the maximum limit, bouncing messages will automatically be forwarded to a stated email address.

Thanks!!

Connect to the mail server on port 110, send some commands:

(you can try this manually in telnet)

POP3:
Return values: +OK=good, -ERR=bad

USER <user name>
PASS <password>
STAT
+OK <# of messages> <total size of messages>
LIST
+OK <# of messages> messages
<message #> <size of message>
<...>
RETR <message #> - get whole message
TOP <message #> <# lines from body> - optional pop command
UIDL [<message #>] - returns a unique ID for the message (optional)
DELE <message #> - mark a message for deletion
RSET - unmark all messages
QUIT - delete marked messages, terminate