How to send email with an attachment from C++ through SMTP?

Question

I'm trying to create an application which will send an email with an attachment. The mail sending works fine, but i can't attach anything. I found this code on the net. Could someone help me, how to attach a file? My code so far:

int sendmail( char * smtpserver, char * from, char * to, char * subject, char * msg )
{
int         iProtocolPort        = 0;
char        szSmtpServerName[64] = "";
char        szToAddr[64]         = "";
char        szFromAddr[64]       = "";
char        szBuffer[4096]       = "";
char        szLine[255]          = "";
char        szMsgLine[255]       = "";
SOCKET      hServer;
WSADATA     WSData;
LPHOSTENT   lpHostEntry;
LPSERVENT   lpServEntry;
SOCKADDR_IN SockAddr;

// Load command-line args
lstrcpyA( szSmtpServerName, smtpserver );
lstrcpyA( szToAddr, to );
lstrcpyA( szFromAddr, from );

// Attempt to intialize WinSock (1.1 or later)
if ( WSAStartup( MAKEWORD( VERSION_MAJOR, VERSION_MINOR ), &WSData ) )
{
    printf( "\nCannot find Winsock v%d.%d or later", VERSION_MAJOR, VERSION_MAJOR );
    return 1;
}

// Lookup email server's IP address.
lpHostEntry = gethostbyname( szSmtpServerName );
if ( !lpHostEntry )
{
    printf( "\nCannot find SMTP mail server %s", szSmtpServerName );
    return 1;
}

// Create a TCP/IP socket, no specific protocol
hServer = socket( PF_INET, SOCK_STREAM, 0 );
if ( hServer == INVALID_SOCKET )
{
    printf( "\nCannot open mail server socket!" );
    return 1;
}

// Get the mail service port
lpServEntry = getservbyname( "mail", 0 );

// Use the SMTP default port if no other port is specified
if ( !lpServEntry ) iProtocolPort = htons( IPPORT_SMTP );
else iProtocolPort = lpServEntry->s_port;

// Setup a Socket Address structure
SockAddr.sin_family = AF_INET;
SockAddr.sin_port   = iProtocolPort;
SockAddr.sin_addr   = *( (LPIN_ADDR)*lpHostEntry->h_addr_list );

// Connect the Socket
if ( connect( hServer, ( PSOCKADDR ) &SockAddr, sizeof( SockAddr ) ) )
{
    printf( "\nError connecting to Server socket!" );
    return 1;
}

// Receive initial response from SMTP server
Check( recv( hServer, szBuffer, sizeof( szBuffer ), 0), "recv() Reply" );

// Send HELO server.com
sprintf_s( szMsgLine, "HELO %s%s", smtpserver, CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() HELO" );
Check( recv( hServer, szBuffer, sizeof( szBuffer ), 0 ), "recv() HELO" );

// Send MAIL FROM: <sender@mydomain.com>
sprintf_s( szMsgLine, "MAIL FROM:<%s>%s", from, CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() MAIL FROM" );
Check( recv( hServer, szBuffer, sizeof( szBuffer ), 0 ), "recv() MAIL FROM" );

char buf[ 128 ], str[ 512 ];
char * pch;

memset( str, 0, sizeof( str ) );

strcpy_s( str, to );

pch = strtok( str, "," );//separate email addresses at ','
while (pch != NULL)
{
    memset( buf, 0, sizeof( buf ) );
    //printf("Buf: %s\n",pch);
    strcpy_s( buf, pch );
    pch = strtok (NULL, ",");

    // Send RCPT TO: <receiver@domain.com>
    sprintf_s( szMsgLine, "RCPT TO:<%s>%s", buf, CRLF );
    Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() RCPT TO" );
    Check( recv( hServer, szBuffer, sizeof( szBuffer ), 0 ), "recv() RCPT TO" );
}

// Send DATA
sprintf_s( szMsgLine, "DATA%s", CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() DATA" );
Check( recv( hServer, szBuffer, sizeof( szBuffer ), 0 ), "recv() DATA" );

//Send Cc
sprintf_s( szMsgLine, "Cc: %s%s", "test@domain.com",CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() Cc" );

// Send Subject
sprintf_s( szBuffer, "Subject: %s\n", subject );
Check( send( hServer, szBuffer, strlen( szBuffer ), 0 ), "send() Subject" );

//Send From
sprintf_s( szBuffer, "From: %s\n", "BIOS Update" );
Check( send( hServer, szBuffer, strlen( szBuffer ), 0 ), "send() From" );

//Send To
sprintf_s( szBuffer, "To: %s\n\n", to );
Check( send( hServer, szBuffer, strlen( szBuffer ), 0 ), "send() To" );

//Attach
sprintf_s( szMsgLine, "Attachment: %s%s", "d:\\test.txt",CRLF );   //this doesn't work
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() Attachment" );

sprintf_s( szMsgLine, "%s%s", msg, CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() message-line" );

// Send blank line and a period
sprintf_s( szMsgLine, "%s.%s", CRLF, CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() end-message" );
Check( recv(  hServer, szBuffer, sizeof( szBuffer ), 0 ), "recv() end-message" );

// Send QUIT
sprintf_s( szMsgLine, "QUIT%s", CRLF );
Check( send( hServer, szMsgLine, strlen( szMsgLine ), 0 ), "send() QUIT" );
Check( recv( hServer, szBuffer, sizeof( szBuffer ), 0 ), "recv() QUIT" );

// Report message has been sent
printf( "\nMail sent!" );

// Close server socket and prepare to exit.
closesocket( hServer );

WSACleanup();

return 0;
}

Thanks!

Answer 1

I suggest you do some reading up on the SMTP protocol.

In the old days the content would be UUENCODED, and merely embedding it in the body of the email with a UUENCODE header was sufficient. I've done that a lot in my time :)

These days everything uses MIME as far as I know. Follow Rup's link (see his comment) or search the internet to understand what MIME does. Searching the internet should also give you two things:

1. Examples of what MIME looks like once the attachment is encoded, and how it is sent on the socket.
2. Resources (libraries etc.) you can use to do the MIME encoding for you, and maybe even shove it down the socket.

You may also find SMTP libraries to handle all the SMTP protocol.

You also need to understand a little more about sockets. Did you honestly expect that writing Attachment: %s%s", "d:\\test.txt",CRLF to a socket would cause the system to open the file and somehow stream the contents of the file to the socket?

Side notes on code:

Why CRLF? All the other email headers send \n to terminate a line. It's all that is needed.

Sending two newlines after an email header (as done for the "To:" header) indicates the end of the email headers. Your "Attachment:" header actually forms the first line of the email body.