I have no trouble sending / receiving emails from other servers, for example gmail.com 

However, I have a specific behaviour with one belgian server : uclouvain.be, where I own an account. 

Sending an email to myadress@uclouvain.be  does not work.  It appears in the troubleshooting window in the list of problematic emails (host not reachable). What's more : the email would bounces back to the user when the system automatically tries to send it (host not reachable, again), but sending the email manually  from the troubleshooting window works  ! I repeated this a couple of times. 

Sending email from myadress@uclouvain.be to the zoho email fails (idem : host not reachable)

Sending and receiving emails from myadress@uclouvain.be to other servers (e.g. : gmail but not only)  works fine.

So : what could explain that uclouvain and zoho just don't want to communicate ? The DNS for my zoho has to servers mx.zoho.com and mx2.zoho.com and I can't figure out what could be wrong.