not receiving emails from one sender

[R.J.Howbrook]

I'm not receiving emails from one specific sender, the rest seem to be coming through fine.

They get a bounce back code with error  '550 5.4.300 message expired-> 451 4.4.400 error communicating with frontend host or destination host - > 421 4.4.2  connection dropped due to socketerror'

i have checked filters and blocked contacts etc and they are all empty