c++ - reading msmtp response from terminal

Question

I am trying to send an email using msmtp SMTP client in Linux from C++ code. Using below code I am able to send out an email successfully. If email is not successful, then error information shown on terminal as shown in below terminal image.

I want to know how can I read error message which is dumpped on terminal using C++ code on Linux?.

int sendmail(const char *to, const char *from, const char *subject, const char *message)
{
	int retval = -1;
	std::string command("/usr/bin/msmtp -t");
	
	try{

		FILE *mailpipe = popen(command.c_str(), "w");
		if (mailpipe != NULL) {
			fprintf(mailpipe, "To: %s\n", to);
			fprintf(mailpipe, "From: %s\n", from);
			fprintf(mailpipe, "Subject: %s\n\n", subject);
			fwrite(message, 1, strlen(message), mailpipe);
			fwrite(".\n", 1, 2, mailpipe);
			retval = pclose(mailpipe); // returns 0 on successful mail sent. else greater than 0 will be given.
		 }
		 else {
			 perror("Couldn't start command. Make sure msmtp installed and configured properly");
		 }
	}
	catch(exception& e)
	{
		cout<<"Error: ";
	}
	 
     return retval;
}