the message is rejected because of CUG visibility restrictions
when there is an internal error for the display message processing (default value in case the reason is not specified in the PHP code)
when the received display message is invalid
when the quota for attachments (size or count) is triggered
when the quota for display messages (size or count) is triggered
When the NDR (negative delivery report) came from the filtering system (antivirus).
when a certain retry operation on the display message is reached
if the original recipient is no more an SmartMS subscriber