Linux下发送邮件附件不超过15K问题

不知道为什么发送的附件低于15K大小是正常，但是大于15K时发送邮件就会socket超时发送失败
怎么附件连1M都达到不了，郁闷中。。。

C/C++ code:

int mail(void){
if(!openMail()){
printf("open mail error\n");
return -1;
}
if(!SendMailGeneral(to,now,subject,"2010-1-16")){
printf("send mail error\n");
return -1;
}
if(SendAffix("test.txt") != 1) {
printf("send affix error\n");
}


if(!End()){
printf("End error\n");
return -1;
}
printf("send over\n");
Destory();

}
/**
* 2.发送邮件
*/

int SendMailGeneral(const char *to,const char *date,const char *subject,const char *data) {
if(to == NULL || date == NULL || subject == NULL) return -1;

sprintf(from,"MAIL from: <%s>\r\n",SERVERMAIL);
rt = send(s,from,strlen(from),0);
if(!GetResponse()) return -1;
sprintf(To,"RCPT TO: <%s>\r\n",to);
rt = send(s,To,strlen(To),0);
if(!GetResponse()) return -1;
sprintf(tmpbuf,"DATA\r\n");
rt = send(s,tmpbuf,strlen(tmpbuf),0);
if(!GetResponse()) return -1;

sprintf(from,"from: %s\r\n",SERVERMAIL);
sprintf(To,"To: %s\r\n",to);
sprintf(Date,"Date: %s\r\n",date);
sprintf(Subject,"Subject: %s\r\n",subject);