Microsoft Exchange 2013 queue is accumulating, where to dig?

Question

Microsoft Exchange

There is a mail server on windows server 2012 r2, exchange 2013 is running on it, at one fine moment incoming letters began to accumulate in the queue, error LED 441 4.4.1
And so everything can hang from half an hour to 2 hours, and then letters leave in 1 second and accumulation starts again.
Please tell me where to dig or who can come across a similar one. Thank you.

Answer 1

The problem is probably on the smart-host side.
Sending to Exchange via smart-host or MX?
PS
Perhaps, by the way, it's just an overloaded external channel to the Internet.