Different answers if single or in a list

Sestotentativo

New Member
We obtain different answers if the email address is a single email or it is in a list.
If the email address is in a list it is considered bad.
The same email in a single email is considered valid.

When the email is in the list the answer is: Service not available - too busy
When the email is single the answer is: OK

If the problem is that the server is too busy I prefer to consider the address valid.
How can I fix it?
 
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