Thank you for reaching out to the Constant Contact Community!
I have taken a look at the engagement for a number of those addresses that you mention and I am not seeing a single successful delivery. I am wondering if these are actually valid addresses as I see they are bouncing as “Undeliverable”. In most cases this means that the address does not actually exist. I don’t think they are blocking Constant Contact emails otherwise the bounce reason would be “Blocked”.
Click Here for more information on our bounce categories. Please let us know if you have additional questions.
Thank you for using Constant Contact, have a great day!
Bit late I know - but I have had this problem for a long time. Some 180 rediffmail addresses bounce regularly. Rediffmail is an Indian service. I have tried to contact someone there with no success. If anyone comes up with a cure would love to hear it.