net-snmp 发送邮件代码 求各位大神解说下

int send_mail(char *addresses, char *sender, 
  char *servers, int SSL, char *mailfile)
{
  int rtn = 0;
FILE *fp = NULL;
//FILE *mail_server_fp = NULL;
auth_context_t authctx = {0};
smtp_session_t session = {0};
smtp_message_t message = {0};
smtp_recipient_t recipient = {0};
const smtp_status_t *status = NULL;
//enum notify_flags notify = Notify_NOTSET;
auth_client_init();
session = smtp_create_session();
message = smtp_add_message(session);

if(SSL){
smtp_starttls_enable (session, Starttls_ENABLED);
}

if (strlen(servers) <= 0) {
//syslog(LOG_ERR, "phone_home: can't get %d mail server address and port", j+1);
syslog(LOG_ERR, "phone_home: can't get mail server address and port");
if (fp != NULL) fclose(fp);
smtp_destroy_session(session);
return 1;
}
smtp_set_server(session, servers);

authctx = auth_create_context();
auth_set_mechanism_flags(authctx, AUTH_PLUGIN_PLAIN, 0);
auth_set_interact_cb(authctx, authinteract_tmp, NULL);
smtp_auth_set_context (session, authctx);
smtp_set_reverse_path(message, sender);

smtp_starttls_set_password_cb(tlsinteract, NULL);
smtp_set_eventcb(session, event_cb, NULL);

smtp_set_reverse_path(message, sender);
smtp_set_header(message, "To", "", "");

fp = fopen(mailfile, "rb");
if (fp == NULL) {
syslog(LOG_ERR, "phone_home: can't open tmp file for sending mail");
  if (fp != NULL) fclose(fp);
smtp_destroy_session(session);
return 1;
}

smtp_set_message_fp(message, fp);

recipient = smtp_add_recipient(message, addresses);

if (smtp_start_session(session) == 0) { 
syslog(LOG_ERR, "phone_home: smtp_start_session for %s failed", addresses); 
if (fp != NULL) fclose(fp);
smtp_destroy_session(session);
return 1;
}

status = smtp_message_transfer_status(message);
rtn = (status->code >= 200 && status->code < 300) ? 0 : 1;
if (rtn == 1) {
syslog(LOG_INFO, "phone_home: mail stauts code %d %s", 
status->code, status->text != NULL ? status->text : ""); 
}

if (fp != NULL) 
fclose(fp);
smtp_destroy_session(session);
auth_destroy_context(authctx);
return rtn;
}
尤其是中间的API函数 不知道什么意思，有这方面经验的大哥大姐给个帮助文档

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