The images used for sending mail are:

  1)  SYS$SYSTEM:MAIL.EXE               local VMS mail (VT interface)
  2)  SYS$SYSTEM:DECW$MAIL.EXE          local VMS mail (X interface)
  3)  SYS$SYSTEM:MAIL_SERVER.EXE        remote VMS mail
  4)  SYS$SYSTEM:PCSA$MAIL_SERVER.EXE   remote PCSA mail
  5)  SYS$LIBRARY:MAILSHR.EXE           used by 1,2,3 and 4
  6)  SYS$LIBRARY:MAILSHRP.EXE          used by 1,2,3,4 and 5
  7)  SYS$LIBRARY:DECW$MAILSHR.EXE      used by 2

Images 1,2,3 and 4 are default installed with SYSPRV if PMDF.

Images 1,2 and 3 are default installed with CMKRNL if PMDF.

See also mail exceeding diskquota.