https://www.tinkercad.com/things/5F7GWggT5m6-franken-pop3/editel?sharecode=NRfYzexSfGseQJBW76hL5OYSSKjJz7xKkWtwtPb-UiQ=

arduino-franken-pop3.cpp

#ifdef ARDUINO
 #include <Arduino.h>
#else
 #include <iostream>
#endif

#include <string.h>

#define YES 1
#define NO 2
	
// Function that printf and related will use to print
int serial_putchar(char c, FILE* f) {
   if (c == '\n') serial_putchar('\r', f);
   return Serial.write(c) == 1? 0 : 1;
}
 
FILE serial_stdout;

//stub
int mainStart();

void setup(){
   Serial.begin(9600);
 
   // Redirect stdout
   fdev_setup_stream(&serial_stdout, serial_putchar, NULL, _FDEV_SETUP_WRITE);
   stdout = &serial_stdout;
   
   printf("This now goes to stdout: %d\n", 3);
   mainStart();
}

void loop(){
    delay(1000);
}

enum cmds
{
    STAT,
    LIST,
    RETR,
    TOP,
    DELE,
    UIDL,
    NOOP,
    RSET,
    CAPA,
    QUIT
};

const char *cmdQuery[QUIT + 1] =
    {
        "STAT", "LIST", "RETR", "TOP", "DELE", "UIDL", "NOOP", "RSET", "CAPA", "QUIT"};

const char *cmdReply[QUIT + 1] =
    {
        "+OK 0 0 STAT", "+OK 0 messages (0 octets) LIST", "-ERR No such message RETR", "-ERR No such message TOP", "-ERR No such message TOP",
        "-ERR No such message UIDL", "+OK NOOP", "+OK 0 messages (0 octets) RSET", "+OK Capability list follows.", "+OK goodbye QUIT"};

int mainStart()
{

    int valid_command = NO;
    char buff[] = "CAPA";
    int activeCmd = QUIT;

    for (int i = 0; i <= QUIT; i++)
    {
        if (strncasecmp(buff, cmdQuery[i], 3) == 0)
        {
            valid_command = YES;
            activeCmd = i;
        }
    }

    if (valid_command == YES)
    {
        switch (activeCmd)
        {
        case STAT:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case LIST:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case RETR:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case TOP:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case DELE:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case UIDL:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case NOOP:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case RSET:
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        case CAPA:
            /*std::cout << cmdReply[activeCmd] << std::endl;
            for (int i = 0; i <= QUIT; i++)
                std::cout << cmdQuery[i] << std::endl;
            std::cout << "." << std::endl;
            */break;
        case QUIT:
            Serial.println( cmdReply[activeCmd] );
            //std::cout << cmdReply[activeCmd] << std::endl; break;
        default:
            break;
        }
    }
    valid_command = NO;
    return 0;
}