This message is provided by anti-spam functionality in the IRC server. In essence it prevents you from sending private messages (text, action, notice, ctcp etc.) to too many different users in a defined period. (I have identified a bug in how this is calculated, so it doesn't work quite as advertised, but the principle is still working.)

Servers that implement this limiting functionality, also usually implement an alternative that is available to operators i.e. CPRIVMSG and CNOTICE. In essence providing that you and the recipient of the message are in a common channel where you are an op, then you can use CPRIVMSG and CNOTICE to send messages without tripping the anti-spam functionality.

Here is some example code that implements this functionality in a friendly way:
Code:
; RPL_iSupport 005 CNOTICE / CPRIVMSG to avoid 439 Target change too frequently message
raw 005:*: { iSupport.Raw005 $1- }
on *:disconnect: { iSupport.Disconnect }

alias -l iSupport.Raw005 {
  ; Remove trailing ":are supported by this server"
  var %new $replace($2-, are supported by this server,$null)
  var %is $iSupport.Name
  var %old [ [ %is ] ]
  var %n = $numtok(%new,$asc($space)), %i 1
  while (%i <= %n) {
    var %n = $gettok(%new,%i,$asc($space)), %d $left(%n,1)
    if (%d == -) %n = $right(%n,-1)
    var %m %n $+ *
    var %j $wildtok(%old,%m,0,$asc($space))
    while (%j) {
      var %o $wildtok(%old,%m,%j,$asc($space))
      if ((%n == %o) || ($+(%n,=*) iswm %o)) %old = $remtok(%old,%o,0,$asc($space))
      dec %j
    }
    if (%d != -) %old = %old $iSupport.Decode(%n)
    inc %i
  }
  set -e [ [ %is ] ] $sorttok(%old,$asc($space),a)
}

alias -l iSupport.Decode {
  return $regsubex(iSupport.Decode,$1,/(\\x[0-9A-Fa-f]{2})/Fg,$chr($base($right(\t,2),16,10)))
}

alias -l iSupport.Disconnect {
  unset [ $iSupport.Name ]
}

alias -l iSupport.Name { return $+(%,ISUPPORT.,$network) }

alias -l iSupport.Supports {
  var %p [ [ $iSupport.Name ] ]
  var %m $1 $+ *
  var %i $wildtok(%p,%m,0,$asc($space))
  while (%i) {
    var %q $wildtok(%p,%m,%i,$asc($space))
    if ($1 == %q) return $true
    if ($+($1,=*) iswm %q) return $deltok(%q,1,$asc(=))
    dec %i
  }
  return
}

; The following provide iSupport equivalents for /msg, /describe, /notice, /ctcp and /ctcpreply
; So that if you are an op you can send frequent messages to other users withourt triggering 
alias -l msg {
  var %c $iSupport.ComChanOp($1)
  if (($iSupport.Supports(CPRIVMSG)) && (%c) && ($left($1,1) !isin $chantypes)) .raw CPRIVMSG $1 %c : $+ $2-
  else .msg $1-
  if ($show) echo -ac Normal -> $+(*,$1,*) $2-
}

alias -l describe {
  .ctcp $1 Action $2-
  if ($show) echo -ac Action -> $+(*,$1,*) $2-
}

alias -l notice {
  var %c $iSupport.ComChanOp($1)
  if (($iSupport.Supports(CNOTICE)) && (%c) && ($left($1,1) !isin $chantypes)) .raw CNOTICE $1 %c : $+ $2-
  else .notice $1-
  if ($show) echo -ac Normal -> $+(-,$1,-) $2-
}

alias -l ctcp {
  var %c $iSupport.ComChanOp($1)
  var %msg $upper($2) $3-
  if (($iSupport.Supports(CPRIVMSG)) && (%c) && ($left($1,1) !isin $chantypes)) .raw CPRIVMSG $1 %c : $+ $iSupport.ctcpEncode(%msg)
  else .ctcp $1-
  if ($show) echo -ac Ctcp -> $+([,$1,]) $upper($2) $3-
}

alias -l ctcpreply {
  var %c $iSupport.ComChanOp($1)
  if (($iSupport.Supports(CNOTICE)) && (%c) && ($left($1,1) !isin $chantypes)) .raw CNOTICE $1 %c : $+ $iSupport.ctcpEncode($2-)
  else .ctcpreply $1-
  if ($show) echo -ac Ctcp -> $+([,$1,]) $2-
}

alias -l iSupport.ctcpEncode {
  var %s $replacex($1-,$chr(16),$+($chr(16),$chr(16)),$chr(0),$+($chr(16),$chr(48)),$chr(10),$+($chr(16),$chr(110)),$chr(13),$+($chr(16),$chr(114)))
  return $+($chr(1),%s,$chr(1))
}

alias -l iSupport.ComChanOp {
  if ($left($1,1) isin $chantypes) return $false
  var %i $comchan($1,0)
  while (%i) {
    if ($comchan($1,%i).op) return $comchan($1,%i)
    dec %i
  }
  return $false
}