 4.     SMTP. POPS. IMAP4_______________181

   SMTP-    -
  ,    SMTP  -
 SMTP  :

1.  , , TCP  UDP,  25.

2. SMTP-.

SMTP-     
SMTP    .     
   SMTP-.

HELO <SP> <domain> <CRLF>

 HELO     ,
 ,   ,     -
 <domain>.    250    .
,        
 SMTP,  :

R: 220 OH-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO AH-XENIX.ARPA
R: 250 OH-UNIX.ARPA

1.       SMTP (  
)     SMTP  .

2.        SMTP-
   .

3.     SMTP    
.

     SMTP,   
QUIT. :

S: QUIT
R: 221 AH-UNIX.ARPA Service closing transmission channel

     SMTP-    
     (. .  SMTP-
),   RSET.   , -
,     .

   , SMTP    ,
      ,     -
          SMTP-
:

 VRFY.        
.      
          -