SMTP（Simple Mail Transfer Protocol）即簡單郵(you)(you)(you)(you)件(jian)傳輸(shu)協議,它是(shi)一(yi)組用于由源地(di)址到目的(de)(de)地(di)址傳送(song)郵(you)(you)(you)(you)件(jian)的(de)(de)規則，由它來控制信(xin)(xin)(xin)件(jian)的(de)(de)中轉方式。SMTP的(de)(de)默認(ren)端(duan)口為：25 .它使用由TCP提供的(de)(de)可靠的(de)(de)數據傳輸(shu)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)把郵(you)(you)(you)(you)件(jian)消息從發(fa)信(xin)(xin)(xin)人的(de)(de)郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)傳送(song)到收(shou)信(xin)(xin)(xin)人的(de)(de)郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)。跟(gen)大多數應用層(ceng)協議一(yi)樣(yang)，SMTP也存在(zai)兩(liang)個(ge) 端(duan)：在(zai)發(fa)信(xin)(xin)(xin)人的(de)(de)郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)上執(zhi)行(xing)(xing)的(de)(de)客(ke)(ke)戶(hu)(hu)端(duan)和(he)在(zai)收(shou)信(xin)(xin)(xin)人的(de)(de)郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)上執(zhi)行(xing)(xing)的(de)(de)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)端(duan)。SMTP的(de)(de)客(ke)(ke)戶(hu)(hu)端(duan)和(he)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)端(duan)同(tong)時運(yun)(yun)行(xing)(xing)在(zai)每個(ge)郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)上。當(dang)一(yi)個(ge)郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu) 務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)在(zai)向(xiang)其他郵(you)(you)(you)(you)件(jian)服(fu)(fu)(fu)(fu)務(wu)(wu)(wu)(wu)(wu)器(qi)(qi)發(fa)送(song)郵(you)(you)(you)(you)件(jian)消息時，它是(shi)作為SMTP客(ke)(ke)戶(hu)(hu)在(zai)運(yun)(yun)行(xing)(xing)。

SMTP(Simple Mail Transfer Protocol)是(shi)(shi)一(yi)種(zhong)(zhong)提供可(ke)靠且有效電子郵件(jian)(jian)傳(chuan)輸的(de)(de)(de)(de)協議。SMTP是(shi)(shi)建模(mo)在(zai)FTP文(wen)件(jian)(jian)傳(chuan)輸服務上的(de)(de)(de)(de)一(yi)種(zhong)(zhong)郵件(jian)(jian)服務,主要用于(yu)傳(chuan)輸系統(tong)之(zhi)間的(de)(de)(de)(de)郵件(jian)(jian)信(xin)息并(bing)提供來(lai)信(xin)有關的(de)(de)(de)(de)通(tong)(tong)(tong)知。SMTP設計基于(yu)以(yi)(yi)下(xia)通(tong)(tong)(tong)信(xin)模(mo)型:針對用戶(hu)的(de)(de)(de)(de)郵件(jian)(jian)請求,發送(song)(song)SMTP與接(jie)(jie)收(shou)(shou)SMTP之(zhi)間建立(li)一(yi)個雙向傳(chuan)送(song)(song)通(tong)(tong)(tong)道。接(jie)(jie)收(shou)(shou)SMTP可(ke)以(yi)(yi)是(shi)(shi)最終(zhong)接(jie)(jie)收(shou)(shou)者也可(ke)以(yi)(yi)是(shi)(shi)中間傳(chuan)送(song)(song)者,即“SMTP郵件(jian)(jian)中繼”。SMTP命令由(you)發送(song)(song)SMTP發出,由(you)接(jie)(jie)收(shou)(shou)SMTP接(jie)(jie)收(shou)(shou),而(er)應答(da)則(ze)反方(fang)面傳(chuan)送(song)(song)。在(zai)因特網(wang)上這個雙向傳(chuan)送(song)(song)通(tong)(tong)(tong)道是(shi)(shi)一(yi)個TCP連接(jie)(jie)。為(wei)SMTP服務分(fen)配的(de)(de)(de)(de)TCP端(duan)口號是(shi)(shi)25。所(suo)以(yi)(yi),可(ke)以(yi)(yi)用“TCPPORT =25”來(lai)判斷一(yi)個TCP流是(shi)(shi)否屬于(yu)一(yi)個SMTP會話,而(er)設置(zhi)包捕(bu)獲時(shi)的(de)(de)(de)(de)過濾規(gui)則(ze)。

SMTP 是(shi)一種提供(gong)可靠且(qie)有效(xiao)電子(zi)郵件(jian)傳輸的協議。 SMTP 是(shi)建(jian)模在 FTP 文件(jian)傳輸服(fu)務上的一種郵件(jian)服(fu)務，主(zhu)要用(yong)于傳輸系統之(zhi)間(jian)的郵件(jian)信(xin)息并提供(gong)來信(xin)有關的通知。

SMTP 獨立于(yu)特定的(de)(de)(de)傳輸(shu)(shu)子(zi)系統，且只(zhi)需要可(ke)靠有序的(de)(de)(de)數據流信道支持。 SMTP 重要特性(xing)之一是其能跨越網(wang)(wang)(wang)絡傳輸(shu)(shu)郵件(jian)(jian)，即“SMTP 郵件(jian)(jian)中繼”。通常，一個網(wang)(wang)(wang)絡可(ke)以(yi)由公(gong)用互聯(lian)網(wang)(wang)(wang)上(shang)(shang) TCP 可(ke)相互訪(fang)問的(de)(de)(de)主機、防火墻分隔(ge)的(de)(de)(de) TCP/IP 網(wang)(wang)(wang)絡上(shang)(shang) TCP 可(ke)相互訪(fang)問的(de)(de)(de)主機，及其它(ta) LAN/WAN 中的(de)(de)(de)主機利用非 TCP 傳輸(shu)(shu)層協(xie)議組成(cheng)。使用 SMTP ，可(ke)實現相同網(wang)(wang)(wang)絡上(shang)(shang)處理機之間的(de)(de)(de)郵件(jian)(jian)傳輸(shu)(shu)，也可(ke)通過中繼器(qi)或網(wang)(wang)(wang)關實現某處理機與其它(ta)網(wang)(wang)(wang)絡之間的(de)(de)(de)郵件(jian)(jian)傳輸(shu)(shu)。

smtp是(shi)(shi)一種傳輸協議！是(shi)(shi)用在電郵上的(de)！你說的(de)smtp服(fu)務器地址(zhi)應(ying)該是(shi)(shi)應(ying)該是(shi)(shi)各個E-mail門戶的(de)網址(zhi)，例如(ru)網易的(de)smtp服(fu)務器地址(zhi)就(jiu)是(shi)(shi)smtp.163.com；新浪的(de)就(jiu)是(shi)(shi)smtp.sina.com