clarification needed : status field in ME22N

Hi all,

I had problem if i send PO Output to an email.

<b><b>ME22N->messages->..i got status Yellow light</b>

If i check the same in scot

Utilities->overview of send orders->

if i execute that screen , i got message in status text like "Message cannot be transferred to node at present due to RFC error"</b>can any body <b>explain reason</b> for this Error?