#5.0.0 smtp; 5.3.0 - Other mail system problem 554-'5.4.6 Message exceeded 50 hops, this may indicate a mail loop

Hi Everyone, 

I'm new here and i've been searching for a while now and i'm not getting any answers from google. 

So the problem is, we cannot send an e-mail to a specific e-mail domain. We are getting this error (see below). We used to send e-mail to this client's address. It was working last week and suddenly this happened. We are using on prem exchange13. I checked the send connectors and I can see that we have 2 enabled connectors and using smart host to route outbound e-mails to mimecast. I hope someone can help me out. 

Remote Server returned '<[64.233.166.27] #5.0.0 smtp; 5.3.0 - Other mail system problem 554-'5.4.6 Message exceeded 50 hops, this may indicate a mail loop