Time a message is queued until a warning email is sent

[Russell Jones]

Hello,

I know that maximal_queue_lifetime is the time a message is queued 
before it is sent back as undeliverable, however what is the 
configuration option for how long a message is attempted to be 
delivered, before a warning message is sent to the original sender 
saying "Hey, this message has been queued for X hours and I haven't been 
able to deliver it."?