You have slow or no email delivery coming from certain domains, and you have a Symantec Messaging Gateway (SMG) behind a Sonicwall appliance.
451 4.4.2 [internal] connection closed by remote host
451 4.4.2 [internal] no HELO/EHLO response
This behavior is commonly caused by SMTP filtering by a Sonicwall firewall device. This can be turned on by default in the Sonicwall appliance due to other features of that appliance.
Sonicwall hardware firewall
It is recommended by Symantec that SMTP filtering by a Sonicwall to the SMG be disabled.
Sonicwall support has documentation available to assist.