This afternoon, I have tried to send several e-mails from both my domains (banatconsultanta.eu and sofiamed.ro)

All e-mail I tried to send, excepting some gmail.com e-mails, did not reach the final recipient.

I received this error:

This message was created automatically by mail delivery system. 

THIS IS A WARNING MESSAGE ONLY. 

YOU DO NOT NEED TO RESEND YOUR MESSAGE. 

The original message was received at Sun, 30 Oct 2016 10:42:03 -0700 
from  cristi@banatconsultanta.eu  [ cristi@banatconsultanta.eu ] 

----- The following addresses had fatal errors ----- 
[Status: Error, Address: < confirmare@digisign.ro >, ResponseCode 421, Host not reachable.] 

Message will be retried for 4 more day(s) 

Should I worry? It is my business e-mail, and I need to be in contact with people.

Regards,

Cristian Mazilu

This message was created automatically by mail delivery system. 

THIS IS A WARNING MESSAGE ONLY. 

YOU DO NOT NEED TO RESEND YOUR MESSAGE. 

The original message was received at Sun, 30 Oct 2016 10:42:03 -0700 
from  cristi@banatconsultanta.eu  [ cristi@banatconsultanta.eu ] 

----- The following addresses had fatal errors ----- 
[Status: Error, Address: < confirmare@digisign.ro >, ResponseCode 421, Host not reachable.] 

Message will be retried for 4 more day(s) 

----- Transcript of session follows ----- 

Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 1477846577318545.3816841413469; Sun, 30 Oct 2016 09:56:17 -0700 (PDT) 
Message-ID:<15816853061.e936 fbfa546610.2582456727249900653@banatconsultanta.eu > 
Date:Sun, 30 Oct 2016 18:56:17 +0200 
From:Cristian Mazilu < cristi@banatconsultanta.eu > 
User-Agent:Zoho Mail 
To:"confirmare" < confirmare@digisign.ro > 
Subject:confirmare reinnoire ANAF - Mazilu Cristian Doru 
Content-Type:multipart/mixed; 
    boundary="----=_Part_1703041_879331719.1477846577258"

OTHER MAIL (sent from @sofiamed.ro)